Mark test source files with a green color

[Kobzol]

This PR improves how the plugin marks test source files:

• The first commit marks files under the tests and benches directories as test files, which means that they will be correctly filtered as tests e.g. in Find in Files -> Scope -> Project Test Files.
• The second commit marks test files with a green color in the Project view (the file list on the left) and also in Find in Usages etc. lists.

TODO/questions:

• Should we make the color configurable/let users disable the coloring?
• The getTestSourceFolders approach seems that it could be slow, should I cache it or something?
• This still doesn't mark usages of tests in Find in XXX that are located in Rust files that are not under tests (like #[cfg(test)] mod tests { ... }. Sadly, the platform only passes the file to the extension point, not the actual Usage instance, so we can't easily find if a specific usage if a test usage. I think that we might be able to solve this by creating some synthetic file for test usages? But it sounds like a horrible hack.

Related issue: #4436

changelog: Mark test source files with a green color in project view and various file lists (like Find in Path).

[Kobzol]

CC @Uriopass, who expressed interest in this feature.