Refactor Harmony tests to better follow Home Assistant conventions

[mkeesey]

Proposed change

This addresses some feedback that was provided in #42331. To support it, there are a couple of high level changes made:

1. I slightly modified the test fixtures we use for creating and injecting our FakeHarmonyClient such that we share a single instance of the client across each test rather than a mocked one that is constructed each time as a side effect. This allows us to capture the callbacks passed into the Harmony client as it's created. Sorry if this is a funky way to do it, but it seemed to be the cleanest way I could figure out. This allows me to move away from getting the data object from hass to simulate hub connections and disconnections, but instead just call methods on my fake client and have those callbacks called appropriately.

2. I mechanically moved the co-mingled tests for switches and remotes from the functionality-specific files (activity changes, etc) into the specific platform files. Generally the new platform tests are now platform specific, but I kept checks in the toggle style tests for the other platform. (e.g. if a switch turns on an activity, ensure the remote shows as on.)

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)
• 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: Generate switches for harmony activities automatically #42331
• Link to documentation pull request: N/A

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.
• Untested files have been added to .coveragerc.

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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 @ehendrix23, @bramkragten, @bdraco, mind taking a look at this pull request as its been labeled with an integration (harmony) you are listed as a codeowner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[bdraco]

Lgtm. Thanks