[Commands] Skip test cases with no tests
[Commands] Skip test cases with no tests

This prevents a crash when a test class doesn't contain any test
methods.

<rdar://problem/42554220> swift test --generate-linuxmain can't handle tests with underscores in the name

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