[tests] Add a test for -iwithprefix.

 - This just scratches the surface, We have pretty horrible test coverage in
   this area it seems like, but this at least covers the change in r173410.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173464 91177308-0d34-0410-b5e6-96231b3b80d8