refactor opts tests
refactor opts tests

- TestParseRunAttach: use subtests to reduce cyclomatic complexity
- TestParseRunWithInvalidArgs: use subtests, and check if the expected
  error is returned.
- Removed parseMustError() as it was mostly redundant

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

thaJeztah/cli,docker/cli,cpuguy83/cli,thaJeztah/cli,cpuguy83/cli,docker/cli