Extend test with a very basic assertion about get_blockdevmgr
Extend test with a very basic assertion about get_blockdevmgr

It isn't a very distinguishing test, but at least it exercises the code.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

trgill/stratisd,stratis-storage/stratisd,trgill/stratisd,stratis-storage/stratisd,mulkieran/stratisd,mulkieran/stratisd,stratis-storage/stratisd