Refactor MQTT tests to use modern platform schema part 1

[jbouwh]

Proposed change

The tests for the MQTT schema rely mainly on yaml the deprecated platform schema. When this 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.

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.

Part 1 covers the specific platform tests (see changed files) and tests from test_init.py.
Part 2 will cover the other platforms: #77525

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

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[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!
_{^{(message by CodeOwnersMention)}}