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