Preemptively add XCTest.xcworkspace

#43 will add a dependency
between swift-corelibs-xctest and swift-corelibs-foundation. On OS X, this
will also necessitate an Xcode workspace that references both XCTest
and Foundation .xcodeproj's.

To prevent CI from breaking when #43 is landed, preemptively add an Xcode
workspace. Before landing #43, the Swift build script will be modified to
build XCTest.xcworkspace, not XCTest.xcodeproj.

Author: modocache