t7527: fix flaky fsmonitor event tests with retry logic

t7527: fix flaky fsmonitor event tests with retry logic

The fsmonitor event tests (edit, create, delete, rename, etc.) were
flaky because there can be a race between the daemon writing events
to the trace file and the test's grep commands checking for them.

Add a retry_grep() helper function (similar to retry_until_success
in lib-git-p4.sh) that retries grep with a timeout, and use it in
all event-checking tests to wait for one expected event before
checking the rest.

Signed-off-by: Paul Tarjan <github@paulisageek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>