MFC r266473,267331,267511:

  Use an intermediate target to associate with _SUBDIR which is marked .MAKE
  this allows make -n to do tree walks as expected without
  doing anything else (as intended).
  Use prefix _sub. to help avoid conflict with any real target.

  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

  Create a mechanism for providing fine-grained build order dependencies
  during SUBDIR_PARALLEL builds.  This augments the coarse .WAIT mechanism,
  which is still useful if you've got a situation such as "almost everything
  depends on A and B".