Enable test_varargs_byval. NFC (emscripten-core#25842)

Seems to work now..

Author: sbc100

test/test_core.py

@@ -11,7 +11,6 @@
 import re
 import shutil
 import time
-import unittest
 from functools import wraps
 from pathlib import Path
 
@@ -2391,7 +2390,6 @@ def test_varargs(self):
   def test_varargs_multi(self):
     self.do_core_test('test_varargs_multi.c')
 
-  @unittest.skip('clang cannot compile this code with that target yet')
   def test_varargs_byval(self):
     src = r'''
       #include <stdio.h>