Add Catch2 unit testing framework + some tests.

Add the Catch2 headers to third_party/include/catch2.

Add Catch.cmake and CatchAddTests.cmake to cmake/.

Add unit tests src/wx/tests/strutils.cpp using Catch2 to test
src/wx/strutils.cpp.

Make some code changes to strutils.cpp to make the tests pass.

See src/wx/tests/CMakeLists.txt for how to set up unit test files; they
plug into the normal CTest mechanism in cmake.

The test binaries are written to the tests/ subdirectory of the build
directory.

Building the tests and enabling the CTest support can be turned off by
passing -DBUILD_TESTING=OFF to cmake, the default is ON, this is the
standard mechanism.

Start running ctest on travis and appveyor.

Move stb to third_party/include where Catch2 is now as well.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

.travis.yml

@@ -85,6 +85,7 @@ script:
         elif [ -n "$XVFB_RUN" ]; then
             xvfb-run -a ./$PRGNAM --help
         fi
+        ctest -V
 
 env:
     global: