Draft: Add random string factory

What does this MR do and why?

Describe in detail what your merge request does and why.

Currently, we use a lot of hard-coded passwords in specs. This is a problem because:

1. A lot of these passwords are week, and we want to implement more stringent password requirements for users. !86310 (merged)
2. We'd like to remove static passwords from our codebase. #360030 (closed)

This change adds a new :random_string factory, which can be used to generate random password to use in specs.

To test this, I tried adding this to a spec and then running it:

diff --git a/ee/spec/controllers/trial_registrations_controller_spec.rb b/ee/spec/controllers/trial_registrations_controller_spec.rb
index dfaba1391a2..76376810dfe 100644
--- a/ee/spec/controllers/trial_registrations_controller_spec.rb
+++ b/ee/spec/controllers/trial_registrations_controller_spec.rb
@@ -82,12 +82,13 @@
         last_name: 'Doe',
         email: 'johnd2019@local.dev',
         username: 'johnd',
-        password: 'abcd1234'
+        password: build(:random_string)
       }
     end
 
     before do
       stub_application_setting(send_user_confirmation_email: true)
+      puts user_params[:password]
     end
 
     subject(:post_create) { post :create, params: { user: user_params } }

Which results in the output:

$ bundle exec rspec ee/spec/controllers/trial_registrations_controller_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Test environment set up in 4.737432 seconds
......x#!@pZ*^PEMziVgPhIf*udlkA1rW4rP%
.&EalsQ4ra6g6oF*OLBIrdu24K8@wxa4p
.QYdpDmG$YHDgY0PUr&9HTrMe9pLUQibQ
.D5W6X5PMr1Os6qZXoHtv9H4oLd*vZ8G4
.10zMevc$xdaeCM)oZVP&8hQ@BE^s7!iZ
.

Finished in 9.33 seconds (files took 9.94 seconds to load)
11 examples, 0 failures

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