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The stack trace suggests LSOpenApplication fails to find the shim, and this
happens after waiting on a run loop. So a likely cause is that we're not
successfully waiting for the file operations to finish.

This CL changes the test to observe the parent directory instead of the shim
directory, and adds checks that the path exists before proceeding. If the test
still fails for this reason, at least it'll say so.

BUG=415422

Review URL: https://codereview.chromium.org/598443002

Cr-Commit-Position: refs/heads/master@{#296185}