selftest: add tests for debugging testing setup

selftest: add tests for debugging testing setup

Many people have developed infrastructure around xfstests. In order to
test a setup, it would be helpful to have dummy tests that have
consistent test outcomes. Add a new test folder with the following
tests:

selftest/001 pass
selftest/002 fail from output mismatch
selftest/003 fail via _fail
selftest/004 skip
selftest/005 crash
selftest/006 hang

Also, create two new groups: 'selftest' which includes tests 001-004 and
'dangerous_selftest' which includes tests 005-006. The selftests will
not run unless explicitly specified.

Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>