Fail lint tests if no tests are found in the test directory

Summary: Ref T4570. Testing a directory with no recognized tests currently passes, but should fail.

Test Plan:
  - Ran `arc unit`.
  - Removed tests from a test directory, ran `arc unit`, got test failure.

Reviewers: leebyron, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4570

Differential Revision: https://secure.phabricator.com/D8434