Generate mocks with ALLOW and EXPECT

[sullerandras]

So the test can be written similar to RSpec

Fixes #. in case of a bug fix, this should point to a bug and any other related issue(s)

Description

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [ ] Includes tests

Reviewer Notes

• [ ] The code flow looks good.
• [ ] Tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- Improved API for custom matchers

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[google-cla[bot]]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
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