Speed up merge request presenter specs

What does this MR do?## What does this MR do?

This MR improves the performance of merge request presenter specs by using let_it_be and before_all where applicable.

Related to &3752.

Contributes to https://gitlab.com/gitlab-org/plan/-/issues/145.

Here's the output from test-prof when run with FPROF=1. Note:

• Total amount of factories created went down from 219 to 18 🚀
• Total events went down from 7515 to 1171
• Queries saved: 6344

Before

[TEST PROF INFO] EventProf results for sql.active_record

Total time: 00:10.822 of 00:25.292 (42.79%)
Total events: 7515

Top 5 slowest suites (by time):

MergeRequestPresenter (./spec/presenters/merge_request_presenter_spec.rb:5) – 00:10.822 (7515 / 51) of 00:25.292 (42.79%)



Finished in 32.56 seconds (files took 27.95 seconds to load)
51 examples, 0 failures

[TEST PROF INFO] Factories usage

 Total: 219
 Total top-level: 171
 Total time: 20.1336s
 Total uniq factories: 6

   total   top-level     total time      time per call      top-level time               name

      54          51       10.4262s            0.1931s             9.6260s            project
      51          51        6.5558s            0.1285s             6.5558s      merge_request
      47          47        2.1603s            0.0460s             2.1603s               user
      45           0        2.1053s            0.0468s             0.0000s          namespace
      21          21        1.7829s            0.0849s             1.7829s              issue
       1           1        0.0086s            0.0086s             0.0086s            license

After

[TEST PROF INFO] EventProf results for sql.active_record

Total time: 00:01.677 of 00:07.915 (21.19%)
Total events: 1171

Top 5 slowest suites (by time):

MergeRequestPresenter (./spec/presenters/merge_request_presenter_spec.rb:5) – 00:01.677 (1171 / 51) of 00:07.915 (21.19%)



Finished in 13.55 seconds (files took 2.1 seconds to load)
51 examples, 0 failures


[TEST PROF INFO] Factories usage

 Total: 18
 Total top-level: 11
 Total time: 3.0930s
 Total uniq factories: 6

   total   top-level     total time      time per call      top-level time               name

       5           2        1.3198s            0.2640s             0.7580s            project
       5           5        0.7960s            0.1592s             0.7960s              issue
       4           0        0.2247s            0.0562s             0.0000s          namespace
       2           2        1.4895s            0.7447s             1.4895s      merge_request
       1           1        0.0111s            0.0111s             0.0111s            license
       1           1        0.0384s            0.0384s             0.0384s               user

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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