[SwiftTestTool] Compute test case list only if necessary.
[SwiftTestTool] Compute test case list only if necessary.

-- https://bugs.swift.org/browse/SR-5334

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