Put the test suite in its own tests.txz distribution file.

Put the test suite in its own tests.txz distribution file.

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