tests(config_flow): add coverage for config exceptions; refactor `ConfigFlow` tests

[palazzem]

Related Issues

n/a

Proposed Changes:

This change improves the testing in the following areas:

1. Removes validate_credentials as it was a level of indirection. This makes testing straightforward as we don't need to test any helper.
2. Adds _() test helper that makes mocking easier with long python paths.
3. Refactors ConfigFlow tests so that they use pytest-mock properly.
4. Fixes pytest-mock by removing context managers as they are not needed.

Testing:

n/a

Extra Notes (optional):

n/a

Checklist

• [x] Related issues and proposed changes are filled
• [x] Tests are defining the correct and expected behavior
• [x] Code is well-documented via docstrings

[coveralls]

Pull Request Test Coverage Report for Build 6746822086

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 89.085%

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Totals
Change from base Build 6746814793:	0.7%
Covered Lines:	476
Relevant Lines:	537

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💛 - Coveralls