Refactor coverage badge spec

What does this MR do?

This MR refactors the Coverage badge spec to be more aligned to the actual public API of the class, and removes some unnecessary work that was effectively testing parts of the Pipeline model that are redundant here.

In addition to being faster, the spec should be much easier to read and understand now.

Screenshots (strongly suggested)

Before

Finished in 9.27 seconds (files took 41.86 seconds to load)
10 examples, 0 failures

[TEST PROF INFO] Factories usage

 Total: 37
 Total top-level: 27
 Total time: 3.7436s
 Total uniq factories: 5

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

      10          10        2.7881s            0.2788s             2.7881s            project
      10           0        0.6493s            0.0649s             0.0000s          namespace
       9           9        0.6612s            0.0735s             0.6612s           ci_build
       7           7        0.2832s            0.0405s             0.2832s        ci_pipeline
       1           1        0.0111s            0.0111s             0.0111s            license

After

Finished in 5.74 seconds (files took 43.95 seconds to load)
8 examples, 0 failures

[TEST PROF INFO] Factories usage

 Total: 6
 Total top-level: 5
 Total time: 0.8988s
 Total uniq factories: 5

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

       2           2        0.2862s            0.1431s             0.2862s           ci_build
       1           1        0.0139s            0.0139s             0.0139s            license
       1           1        0.5281s            0.5281s             0.5281s            project
       1           0        0.1212s            0.1212s             0.0000s          namespace
       1           1        0.0706s            0.0706s             0.0706s        ci_pipeline

For readability, I recommend viewing the old and new versions of the spec file separately, as opposed to a combined diff.

This MR contains !46347 (merged), which should merge first. The changes for review in this MR are all solely in the spec file.

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