Fix running tests by directory

[jhawthorn]

Fixes #21

Previously the code here didn't work, we returned early but with the wrong format. The calling code in neotest interpreted this as "unsupported" and so would itself run tests for every file it knows of.

This implements running multiple files very similar to how Rake::TestTask does, passing the files via ARGV and adding a -e script to require each file passed.

With this we run all test files in the directory in a single process rather than booting once for each file.

[zidhuss]

Thanks for the contributions! 🚀