Split light fixture from implementation to only import when fixture is actually used

[jpbede]

Proposed change

Separate the "fixture" implementation from the fixture to only import when the fixture is actually used instead of always loading it. This avoids tests getting slower when we use this pattern for other tests, thanks @emontnemery for pointing this out on Discord.

Also move the MockLight to tests.components.light.common to follow the common pattern.

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[emontnemery]

Thanks @jpbede 👍

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Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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[emontnemery]

Before merging, I'd like a second opinion on if we prefer the pattern in this PR (which was already used by tts) over the pytest plugin approach.

[MartinHjelmare]

I think this is good. It's a little bit more obvious what's going on.