Refactor MQTT tests to use modern platform schema part 2

[jbouwh]

Proposed change

When the legacy schema is going to be removed with HA 2022.12.0 we need to update the tests to use the modern schema.
This PR will be the start of adopting the the modern schema.

The tests for the MQTT integration rely mainly on yaml the deprecated platform schema. For al platform tests a new DEFAULT_CONFIG will be defined, a clone without the platform attribute.
DEFAULT_CONFIG_LEGACY will hold the default config for the deprecated schema.
A new test will be added temporary to still test setup with the deprecated schema: test_setup_with_legacy_schema.
So all (common)tests that are not converted yet to the new schema will use DEFAULT_CONFIG_LEGACY config until the common code is migrated.
I'll use one commit per platform to begin with. Common tests will be updated later. Finally when the support for config under the platform key is removed all tests referring to the old schema will be labeled with deprecation comment.

This is a follow up for #77387

A PR for the common tests can be found here: #77583

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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[probot-home-assistant]

Hey there @emontnemery, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!
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