Hold the first request until the second is done

The lock tests leaned on a three second sleep in the filter and gave up
the wait for its marker in silence, so on a slow run the requests need
not overlap at all and every check held trivially. A bare wait also
hid any crash of the first request, and the second request's page was
never looked at. The filter now holds the first request until the test
lets it go.

The strace test gets the same hold, released once the trace shows the
second request has opened the lock file. It checks that the delayed
flock then succeeded, since a refusal would mean the inode check was
never reached.

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