Install a symlink from /usr/tests/local to /usr/local/tests.

This is to let Kyua descend into any tests that may have been installed by
ports under /usr/local/tests when running the test suite from /usr/tests.

Some ports (namely those that build Kyua) already install test programs
into /usr/local/tests.  Just make sure to select the TEST option while
building them.

MFC after:	3 days