Remove now-useless ALWAYS_SUBDIRS entry in src/test/Makefile.

Remove now-useless ALWAYS_SUBDIRS entry in src/test/Makefile.

Commit 257836a75 added the "locale" subdirectory to SUBDIRS,
but neglected to remove it from ALWAYS_SUBDIRS.  This oversight
had no functional effect because the filter-out function would
remove it anyway.  Still, it's confusing to readers to list a
subdirectory in both places, especially because it makes the
associated comment into a partial lie.