Extract DRBD related tests

As the DRBD code was extracted from bdev.py, the corresponding tests
should as well be extracted into their own file. That's what this patch
performs.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>