First build that actually works. SDL_main errors before but didn't figure out the cause.

tests/base.cpp

@@ -0,0 +1,9 @@
+#include <catch2/catch_test_macros.hpp>
+#include <fmt/core.h>
+#include <string>
+
+using namespace fmt;
+
+TEST_CASE("base test", "[base]") {
+  REQUIRE(1 == 1);
+}