internal/jsonstream: TestDisplay use streamformatter

[thaJeztah]

• relates to https://github.com/docker/cli/pull/6287
• relates to https://github.com/moby/moby/pull/50759

Similar to 69854c4e08c502f57a1fed4ec58236fae229db37, but for the internal/jsonstream package.

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Ah, derp;

71.02 internal/jsonstream/display.go:1: : import cycle not allowed in test (typecheck)