Force all the contents of /usr/tests to go into a separate distribution
file so that users of binary releases can easily choose to not install it.
To make this possible, we need two fixes:
- bsd.subdir.mk needs to properly honor NO_SUBDIR in all cases so that we
do not recurse into 'tests' subdirectories when we needn't. Otherwise,
we end up with some Kyuafiles in base.txz.
- etc/Makefile needs to skip installing tests in its 'distribute' target
so that a Kyuafile doesn't leak into base.txz.
Approved by: gjb