Make the SCRATCH_DEV optional

Make the SCRATCH_DEV optional

Tests will fail gracefully if there is no SCRATCH_DEV defined, so change

the check script to allow it, and update README to reflect this.

This just makes it simpler to set up the tests, at the expense of
coverage.

Long term it might be nice to allow either SCRATCH_DEV xor TEST_DEV
to be missing, but for now this is simple.

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