[WorkspaceTests] tag only the first entry for in mem mocking
[WorkspaceTests] tag only the first entry for in mem mocking

This makes the behavior similar to localfs package graph mocking

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