run-tests: make sure no_proxy/NO_PROXY are empty to fix test-http-proxy.t

If no_proxy (or NO_PROXY) includes localhost, the test for detecting an
unreachable proxy fails, because the proxy setting is ignored.

tests/run-tests.py

@@ -1129,6 +1129,8 b' def main():'
     os.environ['COLUMNS'] = '80'
     os.environ['GREP_OPTIONS'] = ''
     os.environ['http_proxy'] = ''
+    os.environ['no_proxy'] = ''
+    os.environ['NO_PROXY'] = ''
 
     # unset env related to hooks
     for k in os.environ.keys():