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 tests will come over the next few weeks.  The hope is to have a
simple framework where people can contribute tests for various pieces of
functionality.

  Alan DeKok.