Add a .coffee test proving extension resolve order

[silvenon]

Sending this PR just in case it's useful and connects some dots, feel free to close otherwise.

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Add a test for a more unconventional extension to prove that the extensions rule can take anything as long as the extension resolve order is correct, i.e. it can only remove the extension if it would resolve to the correct file.

I was thinking about documenting this behavior, but it's already documented.

Ref. #965.
Closes #583.

[coveralls]

Coverage remained the same at 96.225% when pulling fdcd4d9 on silvenon:extensions into bc50394 on benmosher:master.