Support custom test paths

[TastyPi]

Motivation / Background

This Pull Request has been created because we're using Packwerk for our Rails app. We'd like to have tests for code in a package be defined in the directory for that package, e.g. packages/a/test. However, there is not a trivial way to make Rails's standard test/test:all etc. commands also run these tests. Our current solution is a custom rake file that redefines the tasks to include the packages' tests, but ideally this would be supported by Rails itself.

Detail

This Pull Request changes Rails::TestUnit::Runner to support specifying default test paths using Rails::Paths::Path. This mirrors how Rails::Engine and Rails::Application can be configured to use different paths from the standard. It also loads config/tests.rb if present, otherwise there is not a good place to configure the paths.

For the example above, the config/tests.rb would contain something like:

Dir.glob("packages/*/test").each do |path|
  Rails::TestUnit::Runner.paths["test"] << path
end

Additional information

This is my first time contributing to Rails, so I suspect this pattern does not fully follow Rails conventions, however I have at least checked that it works. I have not tested it with a project that uses rspec as I am unfamiliar with it.

I would say the most questionable parts are:

1. Is Rails::TestUnit::Runner the right class for this configuration?
2. Is Rails::Paths::Path the best solution for specifying these paths?
3. Should config/tests.rb be introduced or is there a better way to run code before tests that I am unaware of?

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• There are no typos in commit messages and comments.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Feature branch is up-to-date with main (if not - rebase it).
• Pull request only contains one commit for bug fixes and small features. If it's a larger feature, multiple commits are permitted but must be descriptive.
• Tests are added if you fix a bug or add a feature.
  (I have not investigated how to test this and do not wish to spend too much time on it before I know the final implementation and whether this feature is wanted)
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• PR is not in a draft state.
• CI is passing.