add 'terraform test' command

[Gryff]

• Add support for the terraform test command, wrapping it with the same option/argument conventions used by the other command classes.
• Expose it as RubyTerraform.test(...) and RubyTerraform::Commands::Test, supporting :cloud_run, :filter/:filters, :json, :no_color, :test_directory, :vars, :var_file/:var_files, :verbose, and the global :chdir.
• Register four new option definitions (-filter, -cloud-run, -test-directory, -verbose) in the shared definitions table.