run-tests: unset CHGDEBUG

With CHGDEBUG, chg outputs much more stuff and the test could fail running
with --chg. So unset the environment variable.

tests/run-tests.py

@@ -849,7 +849,7 b' class Test(unittest.TestCase):'
         env['TERM'] = 'xterm'
 
         for k in ('HG HGPROF CDPATH GREP_OPTIONS http_proxy no_proxy ' +
-                  'NO_PROXY').split():
+                  'NO_PROXY CHGDEBUG').split():
             if k in env:
                 del env[k]