tests: build tests with C90 standard

C90 mode is disabled for:
- testautomation: use of isnan/isfinite
- testlock: use of kill
- testvulkan: use of c++ style strings in vulkan.h

test/testpopup.c

@@ -180,7 +180,7 @@ static void loop(void)
                         SDL_HideWindow(menus[i].win);
                     }
                 }
-                // Don't process this event in SDLTest_CommonEvent()
+                /* Don't process this event in SDLTest_CommonEvent() */
                 continue;
             }
         }