https://treeherder.mozilla.org/#/jobs?repo=try&revision=9c7e7bb1b1c9138cac7b266dbf29b0e55bb3f9bc&selectedJob=224753195

In the above push the mda1 task has test failures from a test that is normally skipped. I believe this is a regression from bug 1489100 when we started converting directories into a list of tests (to support |mach try coverage|). I assume the mochitest harness has logic to ignore the manifest conditions if you pass in a test file explicitly (which makes sense for debugging purposes). But I haven't validated this theory yet.

Assuming my hunch is correct, I think the best way to fix this is to only expand directories into test paths if we're running |mach try coverage|.

It's also worth noting that this means manifest conditions will also be ignored when running |mach try coverage|. In this case we may need to add some sort of new flag to the test harnesses to make sure we honour skip-if conditions.