[bootstrap] Add support for --test-filter.
[bootstrap] Add support for --test-filter.

 - This is useful for running a small set of tests via `bootstrap test`.

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