Create `sprockets_assets_test.rb` file that will contain the
sprockets-specific tests of `assets_test.rb` (which in turn will contain
the tests common to both pipelines).

There is one flaky test, but I have seen this flake on main before.

```
$ bin/test test/application/sprockets_assets_test.rb

........................E, [2024-09-14T08:35:08.072799 #1983325] ERROR -- : [1ae49fc5-8c5a-48e4-ab4b-0ce5e73f79d0]
[1ae49fc5-8c5a-48e4-ab4b-0ce5e73f79d0] ActionController::RoutingError (No route matches [GET] "/assets/demo.js"):
[1ae49fc5-8c5a-48e4-ab4b-0ce5e73f79d0]
..** Invoke assets:precompile (first_time)
** Invoke assets:environment (first_time)
** Execute assets:environment
** Invoke environment (first_time)
** Execute environment
** Execute assets:precompile
F

Failure:
ApplicationTests::SprocketsAssetsTest#test_precompile_shouldn't_use_the_digests_present_in_manifest.json [test/application/sprockets_assets_test.rb:326]:
Expected "application-0a89120b13dcff9cb069311101add34ef7717de154816b01622972a4509eb5c9.css" to not be equal to "application-0a89120b13dcff9cb069311101add34ef7717de154816b01622972a4509eb5c9.css".

bin/test test/application/sprockets_assets_test.rb:307

..

Finished in 1.424157s, 20.3629 runs/s, 120.0710 assertions/s.
29 runs, 171 assertions, 1 failures, 0 errors, 0 skips

$ bin/test test/application/sprockets_assets_test.rb

........................E, [2024-09-14T08:35:12.994864 #1985849] ERROR -- : [615c9d49-d1bf-40c8-96a6-bf9c76a9a28d]
[615c9d49-d1bf-40c8-96a6-bf9c76a9a28d] ActionController::RoutingError (No route matches [GET] "/assets/demo.js"):
[615c9d49-d1bf-40c8-96a6-bf9c76a9a28d]
..** Invoke assets:precompile (first_time)
** Invoke assets:environment (first_time)
** Execute assets:environment
** Invoke environment (first_time)
** Execute environment
** Execute assets:precompile
I, [2024-09-14T08:35:13.044219 #1986490]  INFO -- : Writing /home/zzak/code/rails/tmp/d20240914-1985782-wcigrx/app/public/assets/rails-6738e33efa7b8d2036e0a0118601555f0b771ac042f6790f7538dd881a1a7f3a.png
I, [2024-09-14T08:35:13.045199 #1986490]  INFO -- : Writing /home/zzak/code/rails/tmp/d20240914-1985782-wcigrx/app/public/assets/application-f1fa81296280307c358b4bd65e4bfd705be90dc5eba7c04dcd1d76ec084c78e7.css
I, [2024-09-14T08:35:13.045508 #1986490]  INFO -- : Writing /home/zzak/code/rails/tmp/d20240914-1985782-wcigrx/app/public/assets/application-f1fa81296280307c358b4bd65e4bfd705be90dc5eba7c04dcd1d76ec084c78e7.css.gz
I, [2024-09-14T08:35:13.045767 #1986490]  INFO -- : Writing /home/zzak/code/rails/tmp/d20240914-1985782-wcigrx/app/public/assets/rails-6738e33efa7b8d2036e0a0118601555f0b771ac042f6790f7538dd881a1a7f3a.png
...

```

Co-authored-by: Tute Costa <tutecosta@gmail.com>
Co-authored-by: Federico Sapuppo <fedesapuppo@hotmail.com>