Fixes some issues i ran into w my recent snap changes:
* Correctly match against patterns that contain subdirectories, eg
`fbt/fbt-call`
* When checking if the input pattern has an extension, only prune known
supported extensions. Our convention of `error.<name>` for fixtures that
error makes the rest of the test name look like an extension to
`path.extname()`.
Tested with lots of different patterns including `error.` examples at
the top level and in nested directories, etc.
627b583650