Convert the remaining 3 authentication tests to MockHTTP.
Convert the remaining 3 authentication tests to MockHTTP.

* test/test_auth.c
  (switched_realm_authn_callback): Update to use dynamic server port number.
  (authentication_switch_realms,
   test_digest_switch_realms,
   test_auth_on_HEAD): Rewrite to use the mock server and stub macro's.

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