[roll] Roll fuchsia [fxfs] Make TestCallback interface simpler.

Previously the callback itself just held a weak that was kept alive by
the strong inside the guard, so dropping the guard would cause no new
callers to find the old callback. However, for this to be true we had to
hold the lock while the callback itself completed. This meant that two
threads could not enter the callback at the same time.

Now the TestCallback holds a normal strong which it clones to be called
outside the lock, but dropping the guard synchronously removes the old
callback.

Now the remaining race is if the guard is dropped after the clone is
made but before it is called, a caller can technically enter the
callback after the guard has been dropped. Since there can always be a
race that they enter the callback but haven't completed any actions
inside the callback then it doesn't matter because both of these two
races are indistinguishable from each other.

This behaviour change does not break any existing uses, as those were
all inside the flush methods to test racing during flushes of a single
file. Flushing is done while holding the truncate lock, so two threads
would never try to enter at the same time.

Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1682794
Original-Original-Revision: dd466e360de68ca815979dbbca7e39442b2aa500
GitOrigin-RevId: ee156b5e100a812f76141efd1d0e51d6800dbd7f
Change-Id: Ie811f13149733fbe608184cf0bd194e7ff935d12
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1684914
Cr-Commit-Position: refs/heads/main@{#195765}