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/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{});
+}