Hey Jon! In order to build and run XCTest, you need the same source layout as required by the Swift project. In other words, you'll need the following folder structure:

some-folder/
    llvm/
    swift/
    swift-corelibs-foundation/
    swift-corelibs-xctest/

In order for the tests to run, they need to find a file named lit.py from within the llvm directory.

Since it seems like a few people are having trouble getting started with contributing to the project, I'll add some additional instructions to the README.

I've hopefully addressed this in #76 which provides clearer instructions on how to get things working. #77 should also help make it clear that building swift-corelibs-xctest requires the same source layout as the Swift project requires.

Thanks for raising this issue, jonallured (JIRA User)! Please create another one if you run into any more trouble.