Speed up API GraphQL specs for Merge Requests

What does this MR do?

This MR speeds up API GraphQL specs for Merge Requests.

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

• Finished in improved by ~60 seconds 🚀
• Total amount of factories created went down from 1012 to 517 🚀

Before

Finished in 2 minutes 32.2 seconds (files took 1.79 seconds to load)
505 examples, 0 failures

Randomized with seed 16771

[TEST PROF INFO] Factories usage

 Total: 1012
 Total top-level: 1009
 Total time: 67.6883s
 Total uniq factories: 8

   total   top-level   total time   top-level time                           name

     505         505      6.9500s          6.9500s          personal_access_token
     498         498     58.6791s         58.6791s                  merge_request
       2           1      0.5609s          0.4016s                        project
       2           0      0.1050s          0.0000s                      namespace
       2           2      1.2942s          1.2942s          labeled_merge_request
       1           1      0.0075s          0.0075s                        license
       1           1      0.1384s          0.1384s                           user
       1           1      0.2175s          0.2175s                          label

After

Finished in 1 minute 26.33 seconds (files took 1.79 seconds to load)
505 examples, 0 failures

Randomized with seed 26268

[TEST PROF INFO] Factories usage

 Total: 517
 Total top-level: 514
 Total time: 8.4844s
 Total uniq factories: 8

   total   top-level   total time   top-level time                           name

     505         505      6.2635s          6.2635s          personal_access_token
       3           3      0.1874s          0.1874s                  merge_request
       2           1      0.5962s          0.4419s                        project
       2           0      0.1112s          0.0000s                      namespace
       2           2      1.2645s          1.2645s          labeled_merge_request
       1           1      0.0069s          0.0069s                        license
       1           1      0.1308s          0.1308s                           user
       1           1      0.1894s          0.1894s                          label

Somehow related to #213649 (closed)

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