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.

zhengcq315/serf,google-code-export/serf,google-code-export/serf,KyleSanderson/serf,zhupan/serf,zhengcq315/serf,stoneCC/serf,zhengcq315/serf,soufianeEL/serf,lazysoez/serf,soufianeEL/serf,x-clone/serf,yesoce/serf,zhupan/serf,KyleSanderson/serf,SirJson/serf,alex1818/serf,google-code-export/serf,KyleSanderson/serf,KyleSanderson/serf,pengdu/serf,soufianeEL/serf,stoneCC/serf,lazysoez/serf,zhengcq315/serf,SirJson/serf,soufianeEL/serf,alex1818/serf,yesoce/serf,pengdu/serf,lazysoez/serf,alex1818/serf,lazysoez/serf,x-clone/serf,pengdu/serf,yesoce/serf,SirJson/serf,stoneCC/serf,zhupan/serf,stoneCC/serf,x-clone/serf,yesoce/serf,google-code-export/serf,SirJson/serf,pengdu/serf,alex1818/serf,x-clone/serf,zhupan/serf