Pass -E to subprocesses from check_add_python_opts

Avoid the runtime environment from affecting the tests' behaviours,
which notably checks the warning filters which can be controlled by
various environment variables.

Author: pradyunsg

Lib/test/test_regrtest.py

@@ -2277,7 +2277,7 @@ def test_python_opts(self):
 
         # Use directly subprocess to control the exact command line
         cmd = [sys.executable,
-               "-m", "test", option,
+               "-E", "-m", "test", option,
                f'--testdir={self.tmptestdir}',
                testname]
         proc = subprocess.run(cmd,