Support comments in the test ignore file (see ../files/test.exclude)

diff -wpruN '--exclude=*.orig' a~/Lib/test/libregrtest/cmdline.py a/Lib/test/libregrtest/cmdline.py
--- a~/Lib/test/libregrtest/cmdline.py	1970-01-01 00:00:00
+++ a/Lib/test/libregrtest/cmdline.py	1970-01-01 00:00:00
@@ -435,6 +435,7 @@ def _parse_args(args, **kwargs):
             ns.ignore_tests = []
         with open(ns.ignore_filename) as fp:
             for line in fp:
+                if line.startswith('#'): continue
                 ns.ignore_tests.append(line.strip())
     if ns.forever:
         # --forever implies --failfast