feat: Allow configuration of test directories and test file pattern

[oskar1233]

In my project I keep test files in lib/ directory next to the code with *.test.exs file names. This pretty much prevented me from using the plug-in in its current state so I made a couple little changes and gathered in this PR.

• expose filter_dir from neotest API directly to config (had no better idea but open to suggestions)
• add test_file_pattern option