Ensure ale_filename_mappings option works optimally

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.