lit: Add a test for discovery w/ test_exec_root (out-of-tree test root).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179401 91177308-0d34-0410-b5e6-96231b3b80d8
lit: Fix infinite recursion when an out-of-tree test root is located inside the source test root.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179402 91177308-0d34-0410-b5e6-96231b3b80d8