Add two new tests for ssltunnel proxy authentication.
Add two new tests for ssltunnel proxy authentication.

* test/test_context.c
  (test_setup_ssltunnel): Fix compiler warning.
  (proxy_authn_callback,
   test_ssltunnel_no_creds_cb,
   test_ssltunnel_basic_auth): New helper function & new tests.
  (test_context): Run the new tests.

blueszhangsh/serf,muthusuba/serf,muthusuba/serf,idomon/serf,muthusuba/serf,krime/serf,liuzongquan/serf,justinweb/serf,wzugang/serf,krime/serf,krime/serf,sworda/serf,muthusuba/serf,justinweb/serf,TommyU/serf,Manouchehri/serf,idomon/serf,TommyU/serf,sworda/serf,Manouchehri/serf,idomon/serf,blueszhangsh/serf,TommyU/serf,blueszhangsh/serf,krime/serf,Manouchehri/serf,wzugang/serf,sworda/serf,TommyU/serf,liuzongquan/serf,idomon/serf,liuzongquan/serf,justinweb/serf,wzugang/serf,Manouchehri/serf,justinweb/serf,wzugang/serf,sworda/serf,liuzongquan/serf,blueszhangsh/serf