Make sure that test covers addition of datadevs after cache is built
Make sure that test covers addition of datadevs after cache is built

Otherwise, certain very significant code paths are not exercised.

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

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