CLI should validate already created testing descriptors

[rafael-direito]

The CLI must support the validation of already created Testing Descriptor. To this end, we must make available a new command line argument --validate-testing-descriptor. When this argument is used, the developers shall pass as input the path to their testing descriptors and the CLI should validate them. If the descriptors are not valid, the CLI should present a list of errors so the developer may correct his descriptor.