fstests: add smoketest group

fstests: add smoketest group

Darrick suggests that fstests can provide a simple smoketest, by
running several generic filesystem smoke testing for five minutes
apiece (SOAK_DURATION="5m"). Since there are only five smoke tests,
this is effectively a 20min super-quick test.

With gcov enabled, running these tests yields about ~75% coverage for
iomap and ~60% for xfs; or ~50% for ext4 and ~75% for ext4; and ~45%
for btrfs.  Coverage was about ~65% for the pagecache.

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