Don't mark diff as "Unit Tests Skipped" when posponed test was skipped

Summary:
Following logic at https://secure.phabricator.com/diffusion/ARC/browse/master/src/workflow/unit/ArcanistUnitWorkflow.php;ea0f737e85644d5a$242-255.

Also unrequire coverage.

Test Plan:

echo '{"diff_id": 1, "file": "empty", "name": "name", "result": "skip", "message": "test"}' | arc call-conduit differential.updateunitresults

Reviewers: nh, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2231