testaudio: Use SDL_SetAppMetadata at startup.

This helps in finding its outputs in system lists of PipeWire streams, etc.

test/testaudio.c

@@ -1060,6 +1060,10 @@ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
 {
     int i;
 
+    char version[32];  /* use SDL's version number, since this test program is part of SDL's sources. */
+    SDL_snprintf(version, sizeof (version), "%d.%d.%d", SDL_MAJOR_VERSION, SDL_MINOR_VERSION, SDL_MICRO_VERSION);
+    SDL_SetAppMetadata("SDL testaudio", version, "org.libsdl.testaudio");
+
     state = SDLTest_CommonCreateState(argv, SDL_INIT_VIDEO | SDL_INIT_AUDIO);
     if (!state) {
         return SDL_APP_FAILURE;