test: enable using lit's internal shell parser
test: enable using lit's internal shell parser

Not all platforms have a full featured shell or a UNIX shell.  Enable
the use of the lit interpreter instead as a shell.

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