Make pry-coolline.gemspec a valid gemspec

The `test/test.rb` file no longer exists and was replaced long ago.
Building the gem locally throws warnings about the gemspec being valid
for that reason. The test files probably don't need to be included in
s.files, but s.test_files should include the real test files.

Signed-off-by: David Celis <me@davidcel.is>