MFC r273803,r273810:

r273803:

  Filter out TESTS_SUBDIRS already added to SUBDIR instead of blindly
  appending the TESTS_SUBDIRS variable to SUBDIR

  Duplicate directory entries can cause unexpected side effects, like
  installing the same files multiple times. This can be easily
  reproduced via the following testcase prior to this commit:

    SUBDIR= dir
    TESTS_SUBDIRS+= dir

    .include <bsd.test.mk>

  Sponsored by: EMC / Isilon Storage Division

r273810:

  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
  Pointyhat to: me (did not use a clean install root)
  Sponsored by: EMC / Isilon Storage Division