Speed up pipeline presenter specs

What does this MR do?

This MR improves the performance of pipeline 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 228 to 37 🚀
• Total events went down from 7677 to 1641
• Queries saved: 6036

Before

[TEST PROF INFO] EventProf results for sql.active_record

Total time: 00:11.392 of 00:23.731 (48.01%)
Total events: 7677

Top 5 slowest suites (by time):

Ci::PipelinePresenter (./spec/presenters/ci/pipeline_presenter_spec.rb:5) – 00:11.392 (7677 / 46) of 00:23.731 (48.01%)



Finished in 29.44 seconds (files took 2.13 seconds to load)
46 examples, 0 failures

[TEST PROF INFO] Factories usage

 Total: 228
 Total top-level: 152
 Total time: 18.1097s
 Total uniq factories: 6

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

      53          46       10.5854s            0.1997s             9.2279s            project
      53           0        2.5169s            0.0475s             0.0000s          namespace
      52          52        2.0841s            0.0401s             2.0841s               user
      46          30        1.4166s            0.0308s             0.9277s        ci_pipeline
      23          23        5.8630s            0.2549s             5.8630s      merge_request
       1           1        0.0070s            0.0070s             0.0070s            license

After

[TEST PROF INFO] EventProf results for sql.active_record

Total time: 00:02.430 of 00:06.481 (37.51%)
Total events: 1641

Top 5 slowest suites (by time):

Ci::PipelinePresenter (./spec/presenters/ci/pipeline_presenter_spec.rb:5) – 00:02.430 (1641 / 46) of 00:06.481 (37.51%)



Finished in 11.98 seconds (files took 9.09 seconds to load)
46 examples, 0 failures

[TEST PROF INFO] Factories usage

 Total: 37
 Total top-level: 22
 Total time: 4.6656s
 Total uniq factories: 6

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

      10          10        3.6072s            0.3607s             3.6072s      merge_request
       9           3        0.3345s            0.0372s             0.1103s        ci_pipeline
       7           7        0.4449s            0.0636s             0.4449s               user
       5           1        1.3331s            0.2666s             0.4950s            project
       5           0        0.2294s            0.0459s             0.0000s          namespace
       1           1        0.0082s            0.0082s             0.0082s            license

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