fixup! mingw_smoketests: skip test_c_ext_build for in-tree builds

This seems to work now, so revert

mingw_smoketests.py

@@ -283,10 +283,6 @@ def test_site(self):
         self.assertEqual(len(site.getsitepackages()), 1)
 
     def test_c_ext_build(self):
-        # This will not work in in-tree build
-        if sysconfig.is_python_build():
-            raise unittest.SkipTest("in-tree build")
-
         import tempfile
         import sys
         import subprocess