Ensure ale_filename_mappings option works optimally

[w0rp]

It's time we do another pass of the codebase to do the following:

1. Simplify code for running linters and fixers so they use the formatted paths and executables everywhere.
2. Simplify tests further.

It may be time to add a command such as AssertFixer to make writing tests for fixers easier, and it may also be time to bite the bullet and update the Vader version we run for tests, as we fixed the version a long time ago.

In addition, we need to run over the LSP code and tsserver code, and make double sure the ale_filename_mappings option is functioning with Docker Compose workflows properly.

[w0rp]

@neersighted This issue might pique your interest!