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

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

include/gsl/multi_span

@@ -41,6 +41,7 @@
 // turn off some warnings that are noisy about our Expects statements
 #pragma warning(push)
 #pragma warning(disable : 4127) // conditional expression is constant
+#pragma warning(disable : 4702) // unreachable code
 
 #if _MSC_VER < 1910
 #pragma push_macro("constexpr")