Add ExtraTests infrastructure

This means
* a new cmake option, `CATCH_BUILD_EXTRA_TESTS`, that conditionally
includes the ExtraTests subfolder
* building and running them on some of the Travis build images
* An example configuration test

In the future these should be extended to cover most of the
configuration options in Catch2, but this is a start.

projects/ExtraTests/CMakeLists.txt

@@ -0,0 +1,29 @@
+#
+# Build extra tests.
+#
+# Requires CATCH_BUILD_EXTRA_TESTS to be defined 'true', see ../CMakeLists.txt.
+#
+
+cmake_minimum_required( VERSION 3.5 )
+
+project( Catch2ExtraTests LANGUAGES CXX )
+
+message( STATUS "Extra tests included" )
+
+# define folders used:
+
+set( TESTS_DIR   ${CATCH_DIR}/projects/ExtraTests )
+set( SINGLE_INCLUDE_PATH ${CATCH_DIR}/single_include )
+
+
+add_executable(FallbackStringifier ${TESTS_DIR}/X10-FallbackStringifier.cpp)
+add_test(NAME FallbackStringifier COMMAND FallbackStringifier -r compact -s)
+set_tests_properties(FallbackStringifier PROPERTIES PASS_REGULAR_EXPRESSION "foo{} for: { !!! }")
+
+
+set_property( TARGET FallbackStringifier PROPERTY CXX_STANDARD 11 )
+set_property( TARGET FallbackStringifier PROPERTY CXX_STANDARD_REQUIRED ON )
+set_property( TARGET FallbackStringifier PROPERTY CXX_EXTENSIONS OFF )
+
+target_include_directories( FallbackStringifier PRIVATE ${SINGLE_INCLUDE_PATH} )
+target_compile_definitions( FallbackStringifier PRIVATE CATCH_CONFIG_FALLBACK_STRINGIFIER=fallbackStringifier )

projects/ExtraTests/ToDo.txt

@@ -0,0 +1,12 @@
+Configuration options that are left default and thus are not properly tested
+yet:
+
+    CATCH_CONFIG_COUNTER                    // Use __COUNTER__ to generate unique names for test cases
+    CATCH_CONFIG_WINDOWS_SEH                // Enable SEH handling on Windows
+    CATCH_CONFIG_FAST_COMPILE               // Sacrifices some (rather minor) features for compilation speed
+    CATCH_CONFIG_DISABLE_MATCHERS           // Do not compile Matchers in this compilation unit
+    CATCH_CONFIG_POSIX_SIGNALS              // Enable handling POSIX signals
+    CATCH_CONFIG_WINDOWS_CRTDBG             // Enable leak checking using Windows's CRT Debug Heap
+    CATCH_CONFIG_DISABLE_STRINGIFICATION    // Disable stringifying the original expression
+    CATCH_CONFIG_DISABLE                    // Disables assertions and test case registration
+    CATCH_CONFIG_DEFAULT_REPORTER

projects/ExtraTests/X10-FallbackStringifier.cpp

@@ -0,0 +1,23 @@
+// X10-FallbackStringifier.cpp
+// Test that defining fallbackStringifier compiles
+
+#include <string>
+
+// A catch-all stringifier
+template <typename T>
+std::string fallbackStringifier(T const&) {
+    return "{ !!! }";
+}
+
+#define CATCH_CONFIG_MAIN
+#include <catch2/catch.hpp>
+
+struct foo {
+    explicit operator bool() const {
+        return true;
+    }
+};
+
+TEST_CASE("aa") {
+    REQUIRE(foo{});
+}