cmake: fix installed SDL3::SDL3_test + test on ci

cmake/test/sdltest.c

@@ -0,0 +1,9 @@
+#include <SDL3/SDL.h>
+#include <SDL3/SDL_test.h>
+
+
+int main(int argc, char *argv[]) {
+    SDLTest_CommonState state;
+    SDLTest_CommonDefaultArgs(&state, argc, argv);
+    return 0;
+}