tests/lit: Change test default parameters to assume local build.  - Also, support overriding them with lit parameters.
tests/lit: Change test default parameters to assume local build.
 - Also, support overriding them with lit parameters.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@168749 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx