Re-add unit tests

Brought test/unit_test.c up to date.

Added a Makefile rule called "unit_test".  Build with "make unit_test" or "make unit_test WITH_LUA=1".  It will produce an executable called "civetweb_test".  That executable will run unit tests.

Unfortunately, I can test only on Ubuntu Linux at the moment.  But since the unit tests didn't work at all before, I don't think it's a huge problem if they still don't work for some platforms, like Windows.