tests: mockito dependency updates due to breaking API changes

[russellwheatley]

Description

mockito dependency breaks the all_plugins/tests check for github actions. It made a breaking change to the API which this PR resolves.

This PR affects auth, core, messaging & firestore packages.

Related Issues

Replace this paragraph with a list of issues related to this PR from the issue database. Indicate, which of these issues are resolved or fixed by this PR. Note that you'll have to prefix the issue numbers with flutter/flutter#.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[github-actions]

Visit the preview URL for this PR (updated for commit 15ea8c0):

https://flutter-firebase-docs--pr5013-russell-mockito-upd-lhlxojtl.web.app

_{(expires Thu, 18 Feb 2021 13:45:50 GMT)}

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[russellwheatley]

Branched off the wrong branch 😢 . Have opened another PR here with the correct base 😄 #5014