Fix Switch not initialized from StateUpdater

[farmio]

by moving state to RemoteValue again

Description

Switch.after_update was not called from RemoteValueSwitch.read_state (StateUpdater). Moved Switch.state to RemoteValue again.
Now it should initialize state correctly from HA again.
I also added a bunch of tests that are mostly useless, but who knows, maybe they will be useful in the future.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• The documentation has been adjusted accordingly
• The changes generate no new warnings
• Tests have been added that prove the fix is effective or that the feature works
• The changes are documented in the changelog
• The Homeassistant plugin has been adjusted in case of new config options

[coveralls]

Coverage remained the same at 94.499% when pulling 250137d on farmio:fix-switch-callback into 73e42c0 on XKNX:master.

[marvin-w]

Looks good to me