Add CMake macros for adding unit test suite files

[pgoodman]

This should just do all the right stuff, and when available, should also generate libFuzzer targets. If we go this route then we can be more certain about the naming of libFuzzer targets (e.g. Foo vs. Foo_LF or Foo.fuzz) and then we can make a wrapper like deepstate-libfuzzer that looks up the right libFuzzer binary.