[WorkspaceTests] Disable a test on linux
[WorkspaceTests] Disable a test on linux

Disabling this test because it intermittently fails on linux. This is
tracked by <rdar://problem/38647101> Intermittent lit test failure. SwiftPMPackageTests. HashedCollections.swift, line 6567

sschiau/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager