The test used `gopkg.in/yaml.v2` to verify the TextMarshaller implementation,
which was implemented to allow printing the errors in JSON formatted output;
> This exists primarily to implement encoding.TextMarshaller such that
> rendering a plugin as JSON (e.g. for `docker info -f '{{json .CLIPlugins}}'`)
> renders the Err field as a useful string and not just `{}`.
Given that both yaml.Marshal and json.Marshal use this, we may as well use
Go's stdlib.
While updating, also changed some of the assertions to checks, so that we don't
fail the test early.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Fairly straight forward. It became necessary to wrap `Plugin.Err` with a type
which implements `encoding.MarshalText` in order to have that field rendered
properly in the `docker info -f '{{json}}'` output.
Since I changed the type somewhat I also added a unit test for `formatInfo`.
Signed-off-by: Ian Campbell <ijc@docker.com>
