fix race in dir_coverage test
fix race in dir_coverage test

Summary:
dir_coverage has been failing fairly repeatably on mac. my theory is that there's a race between the `rm` and the file watcher noticing it before the `batch-coverage` call, so `--wait-for-recheck` doesn't help because no recheck has been started yet.

`force-recheck` is made for this kind of problem and should work on deleted files

Reviewed By: jbrown215

Differential Revision: D26778956

fbshipit-source-id: bd164f49e829b64165994fe45b6fa4b7c22ca698

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow