Increase issues api spec performance by decreasing factory usage

What does this MR do?

• Replaces lets with let_it_bes (small refactors to accomodate this change) to improve test performance.

  TEST PROF report (before):

  [TEST PROF INFO] Factories usage
  
   Total: 1203
   Total top-level: 1199
   Total time: 38.8375s
   Total uniq factories: 17
  
   total   top-level     total time      time per call      top-level time               name
  
     340         340       15.0944s            0.0444s            15.0944s              issue
     178         178        0.6430s            0.0036s             0.6430s         label_link
     152         152        7.8638s            0.0517s             7.8638s       closed_issue
     152         152        1.1235s            0.0074s             1.1235s personal_access_token
     151         151        3.4316s            0.0227s             3.4316s          milestone
     150         150        4.2541s            0.0284s             4.2541s      note_on_issue
      32          32        1.6395s            0.0512s             1.6395s               user
      17          17        0.2170s            0.0128s             0.2170s              label
       9           8        3.0725s            0.3414s             2.3899s      merge_request
       8           7        1.3488s            0.1686s             1.0766s            project
       5           5        0.1869s            0.0374s             0.1869s           assignee
       3           3        0.1364s            0.0455s             0.1364s        award_emoji
       2           0        0.1097s            0.0549s             0.0000s          namespace
       1           1        0.0299s            0.0299s             0.0299s           incident
       1           1        0.7001s            0.7001s             0.7001s merge_requests_closing_issues
       1           1        0.0405s            0.0405s             0.0405s             author
       1           1        0.0103s            0.0103s             0.0103s            license

  TEST PROF report (after):

  [TEST PROF INFO] Factories usage
  
   Total: 297
   Total top-level: 293
   Total time: 8.5905s
   Total uniq factories: 17
  
   total   top-level     total time      time per call      top-level time               name
  
     152         152        1.2262s            0.0081s             1.2262s personal_access_token
      45          45        1.6970s            0.0377s             1.6970s              issue
      29          29        0.1002s            0.0035s             0.1002s         label_link
      17          17        0.9152s            0.0538s             0.9152s               user
      17          17        0.1306s            0.0077s             0.1306s              label
       9           8        2.7367s            0.3041s             2.2618s      merge_request
       8           7        1.1897s            0.1487s             1.0337s            project
       5           5        0.1802s            0.0360s             0.1802s           assignee
       3           3        0.1613s            0.0538s             0.1613s        award_emoji
       3           3        0.2166s            0.0722s             0.2166s       closed_issue
       2           0        0.0776s            0.0388s             0.0000s          namespace
       2           2        0.0630s            0.0315s             0.0630s          milestone
       1           1        0.0364s            0.0364s             0.0364s      note_on_issue
       1           1        0.0361s            0.0361s             0.0361s           incident
       1           1        0.4856s            0.4856s             0.4856s merge_requests_closing_issues
       1           1        0.0358s            0.0358s             0.0358s             author
       1           1        0.0109s            0.0109s             0.0109s            license

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team