tests: Replace returns with exits in the main functions

This will allow us to redefine the exit function.

For example:
```
#define exit(code) { \
    fprintf(stderr, "%s:%d: exit(%s) in function ‘%s’\n",
        __FILE__, __LINE__, #code, __func__); \
    _exit(code); \
}
```

tests/stdio/sprintf.c

@@ -12,7 +12,7 @@ int main(void) {
     );
     if (ret != 68) {
         printf("Failed! Return value was %d\n", ret);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
 
     memset(buffer, 0, sizeof(buffer));
@@ -25,7 +25,7 @@ int main(void) {
     );
     if (ret != 86) {
         printf("Failed! Return value was %d\n", ret);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
 
     puts(buffer);