adds a google_cloud.instances mock to conftest

[dcmcand]

Reference Issues or PRs

Related to #2730

What does this implement/fix?

adds google_cloud.instances mock to conftest to allow the tests in #2730 to pass

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

How to test this PR?

CI Passes

Any other comments?