Chore: Re-organise tests

[w0rp]

I want to re-organise the test files a bit to make some filenames simpler and put all of the additional test files into one place so they don't clutter the Vader test files directories as much. The new structure should look like so:

test/vimrc           # The vimrc for running tests in Docker
test/script/         # Scripts for running tests
test/test-files/     # All dummy files for checking path searching go in here
test/linter/         # Linter command tests go in here
test/fixer/          # Fixer command tests go in here
test/handler/        # Linter problem handlers go in here
test/lsp/            # LSP feature tests go in here
test/...             # Other directories and .vader files...

I want to simplify the Vader filenames. Linter tests like test_zig_zls_callbacks.vader should be named test_zig_zls.vader instead, and fixer tests like test_terraform_fmt_fixer_callback.vader should be named test_terraform_fmt.vader instead. The handler tests can retain their _handler.vader filenames so they are easy to tell apart from linter command tests just by looking at the base names of the test files.

[w0rp]

@hsanson I'll try to finally complete this for 4.0.0. I'm also considering doing a trick with find where we automatically determine the depth .vader files lie at and use that to generate the glob arguments for .vader files. We need to pass globs for test files because the command line string will be too long if we specify each individual file, but I can imagine the Bash script code in my head that would generate any depth of globs using only * and not **. I'll avoid ** so the scripts still work with Bash 3. (Mac OSX...)

[w0rp]

This should be pretty much the structure we use now.