[SwiftTestTool] Do not run parallel test runner when there are zero matches
[SwiftTestTool] Do not run parallel test runner when there are zero matches

- <rdar://problem/34494793> [SR-5917]: swift-test gets stuck in parallel mode when --filter doesn't match any test case

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