externals: Add catch2 v3.2.1

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

externals/catch/examples/020-TestCase-1.cpp

@@ -0,0 +1,29 @@
+// 020-TestCase-1.cpp
+
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE( "1: All test cases reside in other .cpp files (empty)", "[multi-file:1]" ) {
+}
+
+// ^^^
+// Normally no TEST_CASEs in this file.
+// Here just to show there are two source files via option --list-tests.
+
+// Compile & run:
+// - g++ -std=c++14 -Wall -I$(CATCH_SINGLE_INCLUDE) -c 020-TestCase-1.cpp
+// - g++ -std=c++14 -Wall -I$(CATCH_SINGLE_INCLUDE) -o 020-TestCase TestCase-1.o 020-TestCase-2.cpp && 020-TestCase --success
+//
+// - cl -EHsc -I%CATCH_SINGLE_INCLUDE% -c 020-TestCase-1.cpp
+// - cl -EHsc -I%CATCH_SINGLE_INCLUDE% -Fe020-TestCase.exe 020-TestCase-1.obj 020-TestCase-2.cpp && 020-TestCase --success
+
+// Expected test case listing:
+//
+// prompt> 020-TestCase --list-tests *
+// Matching test cases:
+//   1: All test cases reside in other .cpp files (empty)
+//       [multi-file:1]
+//   2: Factorial of 0 is computed (fail)
+//       [multi-file:2]
+//   2: Factorials of 1 and higher are computed (pass)
+//       [multi-file:2]
+// 3 matching test cases