Simplify and demuck some of the linter test cases

Summary:
Ref T3186.

  - Every linter builds a WorkingCopyIdentity in the same way, with no specialized data. Don't do that.
  - Linters get passed a goofy hardcoded ".php" path. Don't do that.
  - Linters generally run on an imaginary path, which might not work. Just give them a real path by building a tiny working copy in `/tmp`.
  - Fix a TODO now that we have better typechecking.

Test Plan: `arc unit --everything`, intentionally broke a test to make sure that still works.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3186

Differential Revision: https://secure.phabricator.com/D6798