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/realpath.c

@@ -8,7 +8,7 @@ int main(void) {
     char* path = realpath("stdlib/realpath.c", NULL);
     if (!path) {
         perror("realpath");
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     puts(path);
 
@@ -21,7 +21,7 @@ int main(void) {
     if (!path) {
         perror("realpath");
         free(path);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     puts(path);