Fix: Find file pattern

The pattern used to find lua files in the test directory is not
correct and also matches some non-lua files like vim cache files.

I had two kind of artifacts matched that should have: .lua.swp or .lua~
files but I guess there can be more.

The issue isn't big in a sense since it doesn't break bourbon but
still it prints some annoying error messages on the output.
Merge pull request #14 from pauldub/fix/find-file-pattern

Fix: Find file pattern