TestURLSession: Fix test_concurrentRequests()
TestURLSession: Fix test_concurrentRequests()

- Add a timeout to the DispatchGroup wait().

- Re-enable the test.

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation