add freeze and dangerous groups

add freeze and dangerous groups

Add 2 new test groups:

freeze: tests which test filesystem freeze
dangerous: tests which may hang or oops

The 2nd may be useful for automated testing to do i.e.

./check -g auto -x dangerous
./check -g auto,dangerous

to try to get fuller coverage before running into tests
which may panic or hang the box and stop the test cycle.

I doubt I have all the potential dangerous tests, but
they can be added later when found.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>