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/stdlib/env.c

@@ -34,7 +34,7 @@ int main(void) {
     if (env) {
         puts("This should be null, but isn't");
         puts(env);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     } else {
         puts("Value deleted successfully!");
     }