Move get_mock_context method to central utils file.

[purple4reina]

What does this PR do?

Moves get_mock_context to a new utils.py file.

Motivation

There was a get_mock_context method in almost every test file.

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
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[duncanista]

Thanks for adding the utils!