generic/476: reclassify this test as a long running soak stress test

generic/476: reclassify this test as a long running soak stress test

This test is a long(ish) running stress test that uses fsstress, so
alter its group membership as follows:

long_rw: because this can read and write to the fs for a long period of
time

stress: because this test employs fsstress

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>