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