tests: replace custom MockConfigEntry with the official `hass.MockConfigEntry`

[palazzem]

Related Issues

n/a

Proposed Changes:

Instead of using our own MockConfigEntry, we're using the one provided directly from HA. It behaves exactly as the original one, with the only difference that we remove the MappingProxyType mapping (read-only protection) as we want to override values during tests.

This change is required to use config_entry fixture while testing our OptionsFlow.

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 6747972151

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.085%

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Totals
Change from base Build 6746931046:	0.0%
Covered Lines:	476
Relevant Lines:	537

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💛 - Coveralls