externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c598a9a517d7bdec8ba1a0bace3ef10' as 'externals/catch'

externals/catch/fuzzing/NullOStream.cpp

@@ -0,0 +1,10 @@
+#include "NullOStream.h"
+
+void NullOStream::avoidOutOfLineVirtualCompilerWarning()
+{
+}
+
+int NullStreambuf::overflow(int c){
+    setp(dummyBuffer, dummyBuffer + sizeof(dummyBuffer));
+    return (c == traits_type::eof()) ? '\0' : c;
+}