Fix help text for plugins

When using `CommandDisplayNameAnnotation` we want to use it instead of
the command name in `--help` message or in the default help command.

With current code we get the wrong text in the --help usage text:

    Flags:
      -h, --help   help for kubectl-plugin

And in the long description of the default help command:

    $ kubectl cobraplugin help -h
    Help provides help for any command in the application.
    Simply type kubectl-plugin help [path to command] for full details.

The issue was hidden since the test checked only the Usage line.

Fixed by extracting a displayName() function and use it when creating
FlagSet and when formatting the default help flag usage and the help
command long description.

Enhance the TestPlugin to check all the lines including the command
name.