Move from unittest-cpp to catch for unit testing. (#533)

Many thanks to @rianquinn. This should fix #495, #494 and #529.

include/gsl/span

@@ -37,6 +37,7 @@
 
 // turn off some warnings that are noisy about our Expects statements
 #pragma warning(disable : 4127) // conditional expression is constant
+#pragma warning(disable : 4702) // unreachable code
 
 // blanket turn off warnings from CppCoreCheck for now
 // so people aren't annoyed by them when running the tool.