externals: Add catch2 v3.2.1

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

externals/catch/tests/ExtraTests/X10-FallbackStringifier.cpp

@@ -0,0 +1,35 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+/**\file
+ * Test that the user can define custom fallbackStringifier 
+ *
+ * This is done by defining a custom fallback stringifier that prints
+ * out a specific string, and then asserting (to cause stringification)
+ * over a type without stringification support.
+ */
+
+#include <string>
+
+// A catch-all stringifier
+template <typename T>
+std::string fallbackStringifier(T const&) {
+    return "{ !!! }";
+}
+
+#include <catch2/catch_test_macros.hpp>
+
+struct foo {
+    explicit operator bool() const {
+        return true;
+    }
+};
+
+TEST_CASE("aa") {
+    REQUIRE(foo{});
+}