Add test for Basic authentication.
Add test for Basic authentication.

* test/test_context.c
  (authn_callback,
   test_basic_authentication): New helper function and test.
  (test_context): Run the new test.

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