Make arcanist lint tests flexible about testcase formatting

Summary:
We currently require an exact number of "~" characters, but this is needless and error-prone.

Instead, allow any number larger than 3.

Test Plan:
  - Added meta test-case.
  - Ran `arc unit --everything`.

Reviewers: btrahan, thoughtpolice

Reviewed By: thoughtpolice

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10251