test: Add a mode to skip tests that involve arbitrary delays

This can be used as a quick acceptance test for CI workflows.

Signed-off-by: Simon McVittie <smcv@collabora.com>

test/testthread.c

@@ -91,6 +91,12 @@ main(int argc, char *argv[])
         return (1);
     }
 
+    if (SDL_getenv("SDL_TESTS_QUICK") != NULL) {
+        SDL_Log("Not running slower tests");
+        SDL_Quit();
+        return 0;
+    }
+
     while (argv[arg] && *argv[arg] == '-') {
         if (SDL_strcmp(argv[arg], "--prio") == 0) {
             testprio = 1;