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