Build an index store for test targets if test discovery is enabled
Build an index store for test targets if test discovery is enabled

Test discovery requires an index store for the test targets to discover
the tests.

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