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