Fix the logic inversion in the previous commit by ensuring that the matched
expression (:M) is empty, not the not matched (:N) is empty. The former case
means we have not found the TEST_SUBDIR value in SUBDIR

Reported by: rodrigc
X-MFC with: r273803
Pointyhat to: me (did not use a clean install root)
Sponsored by: EMC / Isilon Storage Division