Fix flaky test "#protected_branch_exists?"

What does this MR do and why?

Contributes to #470324 (closed)

Call to all_protected_branches is cached and not reloaded between consecutive tests.

let_it_be_with_reload forces project to refresh associations.

MR acceptance checklist

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Screenshots or screen recordings

Before

bundle exec rspec --format documentation spec/services/projects/protect_default_branch_service_spec.rb:259
Run options: include {:focus=>true, :locations=>{"./spec/services/projects/protect_default_branch_service_spec.rb"=>[259]}}

Test environment set up in 2.311242 seconds

Projects::ProtectDefaultBranchService
  #protected_branch_exists?
    when feature flag `group_protected_branches` disabled
      return false
    when feature flag `group_protected_branches` enabled
      return true (FAILED - 1)

Failures:

  1) Projects::ProtectDefaultBranchService#protected_branch_exists? when feature flag `group_protected_branches` enabled return true
     Failure/Error: expect(service.protected_branch_exists?).to eq(true)

       expected: true
            got: false

       (compared using ==)

       Diff:
       @@ -1 +1 @@
       -true
       +false

After

bundle exec rspec --format documentation spec/services/projects/protect_default_branch_service_spec.rb:259
Run options: include {:focus=>true, :locations=>{"./spec/services/projects/protect_default_branch_service_spec.rb"=>[259]}}

Test environment set up in 2.383773 seconds

Projects::ProtectDefaultBranchService
  #protected_branch_exists?
    when feature flag `group_protected_branches` disabled
      return false
    when feature flag `group_protected_branches` enabled
      return true

Finished in 6.27 seconds (files took 17.19 seconds to load)
2 examples, 0 failures

How to set up and validate locally

1. bundle exec rspec --format documentation spec/services/projects/protect_default_branch_service_spec.rb:259