externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c' as 'externals/catch'

externals/catch/fuzzing/NullOStream.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <ostream>
+#include <streambuf>
+
+// from https://stackoverflow.com/a/8244052
+class NullStreambuf : public std::streambuf {
+  char dummyBuffer[64];
+
+protected:
+  virtual int overflow(int c) override final;
+};
+
+class NullOStream final : private NullStreambuf, public std::ostream {
+public:
+  NullOStream() : std::ostream(this) {}
+  NullStreambuf *rdbuf() { return this; }
+  virtual void avoidOutOfLineVirtualCompilerWarning();
+};
+