Revert "[xctest] disable the test due to swiftlang/swift-corelibs-foundation#304"

Author: shahmishal

test-xctest-package/test-xctest-package.py

@@ -1,8 +1,6 @@
 # Trivial test for importing XCTest.
 #
-# https://github.com/apple/swift-corelibs-foundation/pull/304
-# REQUIRES: disabled
-#
+
 # This test doesn't work on Darwin yet, because the XCTest overlay isn't shipped
 # with the package, and can't be found:#
 #   <rdar://problem/23600043> Cannot import XCTest with swift from a downloadable package