Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

test/testtimer.c

@@ -80,7 +80,7 @@ int main(int argc, char *argv[])
     Uint64 start, now;
     Uint64 start_perf, now_perf;
     SDLTest_CommonState  *state;
-    SDL_bool run_interactive_tests = SDL_TRUE;
+    bool run_interactive_tests = true;
     int return_code = 0;
 
     /* Initialize test framework */
@@ -96,7 +96,7 @@ int main(int argc, char *argv[])
         consumed = SDLTest_CommonArg(state, i);
         if (!consumed) {
             if (SDL_strcmp(argv[i], "--no-interactive") == 0) {
-                run_interactive_tests = SDL_FALSE;
+                run_interactive_tests = false;
                 consumed = 1;
             } else if (desired < 0) {
                 char *endptr;