Cover all test cases for update-deps with automatic tests.

[sculpt0r]

Extracted from: https://github.com/ckeditor/ckeditor4-workflows-common/pull/15 with comment.

update-deps can be invoked in many different situations:

• there is no package.json at the target branch
• dependencies might be updated
• dependencies might be old and needs an update
• there is or there is not a branch for PR

[f1ames]

There is already a PR in progress covering that issue, see https://github.com/ckeditor/ckeditor4-workflows-common/pull/28.

[sculpt0r]

❄️ Let's wait with this issue - until we decide if the current bot-updates flow is really unpleasant.