Separate `testing` in `@fedify/fedify` to `@fedify/fixture`

[2chanhaeng]

Summary

Separate testing in @fedify/fedify to @fedify/fixture.

Target Area

• packages/fedify

Motivation

While working on #437, I found that there is a heavy dependency on the testing module. Therefore, it is necessary to separate this module into its own package.
But publishing is unnecessary.

Risk Assessment

No response

Benefits

It will be useful for tests when separating packages including tests from @fedify/fedify or creating tests for other packages.

[dahlia]

Tip: To prevent publishing, you can add the following settings:

• "private": true in package.json → prevents npm publish
• "publish": false in deno.json → prevents JSR publish