Precompiled assets with digest strings are ignored in CI

While debugging gitlab-org/gitlab-ce!14533 I ran across a strange revelation. While we currently pre-compile our sprockets-managed static assets in the prepare CI stage, these assets are not actually served in our RSpec feature tests.

Assets with digest strings (e.g. application-8c49f0a0dd1c10e71c936541134af1f0b9a8faaa13b70dbe86e33d8b2a0cbf1d.css) are being generated, but the asset paths being output in our test environment don't match them (e.g. application.css). This causes sprockets to re-generate them on-demand, potentially wasting a ton of time in our later CI steps.

From the original discussion:

the pre-compiled assets from the prepare CI step were being ignored (except the webpack ones which use their own manifest) because when the sprockets assets:precompile task is run, digests are always included in the filenames regardless of the config.assets.digest setting, but when asset paths were generated in our haml views using rails helpers, the config.assets.digest = false setting was honored meaning the precompiled assets have digest strings and the generated paths did not.

the reason the tests didn't all blow up before is because config.assets.compile = true for development and test environments, meaning sprockets will see a request for something like /assets/application.css and when the asset doesn't already exist it will lazily compile it on-demand.

I've removed the config.assets.digest = false setting and added config.assets.compile = false for the test environment when run in CI. This means it should now use the precompiled assets instead of generating them on-demand (this might just save a TON of cpu cycles in our rspec test suite!)
The errors we're now seeing in the pipeline are due to tests which were written presuming digest strings do not exist. e.g.
  1) Projects::JobsController GET status.json return a detailed job status in json
 Failure/Error: expect(json_response['favicon']).to eq "/assets/ci_favicons/#{status.favicon}.ico"

   expected: "/assets/ci_favicons/favicon_status_pending.ico"
        got: "/assets/ci_favicons/favicon_status_pending-5ecc554aa46a62bdb293a698e37f1fa6e2eda95f130ef8a311a5d800147ba257.ico"
I think we ought to update these failing tests manually, to account for the presence of digest strings. The potential performance improvement vastly outweighs the inconvenience of fixing these tests.

Solution:

We remove config.assets.digest = false from our test environment config, and we conditionally add config.assets.compile = false when it is run in CI. This will ensure that:

Assets will not be compiled on-demand (and errors will be thrown for malformed asset paths)
Asset paths will correctly match those generated in the precompile process

A few tests which assumed non-digest paths will need to be corrected, but this should not be a problem.