Unit tests in 3.0

[Alan DeKok]

  3.0 and master now have the start of a unit test framework, in
src/tests/unit/.  Parts were there before, but it appears they weren't run.

  The build now refuses to link radiusd if the unit tests fail.

  More unit tests are now in src/tests/keywords/.  See the README in
that directory for documentation on how the tests work.

  We should now be able to have a large number of small tests.  Each
test should test one piece of the "unlang" framework.  The net result
should be that there are fewer regressions in functionality.

  Alan DeKok.