Previously it would take seconds to render the pipeline index JSON
(/gitlab-org/gitlab/pipelines.json) in the GitLab project. Apparently,
we render over 5000 paths for `BuildArtifactEntity`, and generation of
these paths dominated the request time.

The Rails path helpers are slow because they check the namespace and
project parameters against large regular expressions, which include
`TOP_LEVEL_ROUTES` and `PROJECT_WILDCARD_ROUTES`, respectively.

We can speed this up by generating the paths directly with
strings. While this isn't ideal, this results in a 10x improvement in
speed:

```
Calculating -------------------------------------
                orig    86.000  i/100ms
                 new     1.272k i/100ms
-------------------------------------------------
                orig      1.343k (±13.8%) i/s -      6.536k
                 new     13.756k (±15.3%) i/s -     67.416k

Comparison:
                 new:    13756.3 i/s
                orig:     1343.3 i/s - 10.24x slower
```

To prevent these helper methods from breaking if the routes ever change,
we add tests to ensure that the results are the same.

Closes https://gitlab.com/gitlab-org/gitlab/issues/121929