fstests: mark tests that are unreliable when run in parallel

fstests: mark tests that are unreliable when run in parallel

Add a group named "unreliable_in_parallel" to mark tests that
do not give reliable results when multiple tests are run in
parallel. Generally this happens with tests that are reliant on
caching in some way, such as generating specific file layouts using
buffered IO or expecting inodes to be cached in memory. These are
perturbed by other tests running sync(), generating memory pressure,
dropping caches, etc.

Hence whether these tests pass or fail is wholly dependent on what
tests are running at the same time, and hence randomly fail when
nothing has actually gone wrong. Hence they are unreliable as
regression tests when running tests in parallel, so we add them to
the "unreliable_in_parallel" group and a parallel check can exclude
this group.

As tests are updated to be robust against external interference,
they can be removed from the unreliable_in_parallel group.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>