[fxfs] Make profile tests timeout properly.

Instead of relying on the overall test timeout, use an async timeout on
the polling loops so that we can be sure that's what is failing and not
some shutdown part of the profile getting wedged.

Moves from checking the `committed_bytes` to `populated_bytes`, which
overall shouldn't matter in our case but is the more general measure of
pages available in the VMO instead of fully committed pages.

Bug: 484869733
Change-Id: If55587caeffe674250a2072780ce672a3a454fd7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1552862
Commit-Queue: Martin Lindsay <mlindsay@google.com>
Reviewed-by: Stephen Demos <sdemos@google.com>