mdadm/test: Mark and ignore broken test failures

mdadm/test: Mark and ignore broken test failures

Add functionality to continue if a test marked as broken fails.

To mark a test as broken, a file with the same name but with the suffix
'.broken' should exist. The first line in the file will be printed with
a KNOWN BROKEN message; the rest of the file can describe the how the
test is broken.

Also adds --skip-broken and --skip-always-broken to skip all the tests
that have a .broken file or to skip all tests whose .broken file's first
line contains the keyword always.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>