test: use `lit` to provide `which`
test: use `lit` to provide `which`

Rather than attempting to implement the utility finding by hand, which
does not account for builtin utilities, use `lit` properly.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild