Derive the right paths to use during testing instead of passing it in via make.

Derive the right paths to use during testing instead of passing it in via make.

Also, fix a few other details of the cmake test target and rename it to
'check'. CMake tests now work for the most part, but there are a handful of
failures left due to missing site.exp bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86452 91177308-0d34-0410-b5e6-96231b3b80d8