Rewrite FileFinder tests to actually show why they break

Summary:
Depends on D18979. Currently, when you break FileFinder you get "test failure! got true, expected false on some random line".

There's no way to see why your change broke stuff, since the assertions are all these very narrow key checks in a result list.

Just test that the entire result list is what we expect so there's a nice diff in the output if you break something. Also, simplify the code.

Test Plan: arc unit is still clean, got sensible test errors while writing this.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18980