[lit] Fix unit test discovery for Visual Studio builds.

Fix by Andrew Ng!

The Visual Studio build can contain output for multiple configuration types (
e.g. Debug, Release & RelWithDebInfo) within the same build output
directory. Therefore when discovering unit tests, the "build mode" sub directory
containing the appropriate configuration is included in the search. This sub
directory may not always be present, so a test for its existence is required.

Reviewers: zturner, modocache, dlj

Reviewed By: zturner, dlj

Subscribers: grimar, bd1976llvm, gbreynoo, edd, jhenderson, llvm-commits

Differential Revision: https://reviews.llvm.org/D34976

llvm-svn: 307235