Use freezegun in DST tests

[emontnemery]

Proposed change

Use freezegun in some DST related tests

Includes changes from spulec/freezegun#424

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[bdraco]

Looks good.

Should fix the failure seen here: https://github.com/home-assistant/core/runs/4082393018?check_suite_focus=true

[cgtobi]

Can we now start using freezegun for other tests as well? Because last time I tried to introduce freezegun @balloob did not like it. Don't get me wrong, I am all for it.

[bdraco]

Can we now start using freezegun for other tests as well? Because last time I tried to introduce freezegun @balloob did not like it. Don't get me wrong, I am all for it.

Not sure. I wouldn't have approved this one knowing that. These tests have been a constant source of flapping CI failures so we need a solution here even if we decide freezegun isn't the right one.

[cgtobi]

For reference here is his comment on that PR of mine. #46392

[emontnemery]

I discussed this PR with @balloob yesterday before merging, and he's OK with it.

In general, patching homeassistant.util.dt.utcnow is error-prone as we use other methods than that to get the time, and it may be imported under a different name so we end up not patching it everywhere.
In particular, the DST tests were prone to breaking because they failed when trying to set a date in the past, so we instead intended to set them to DST the next year. Before the big DST issue hit, they had been broken for several years because the DST date had not been bumped 🤦

Since we're now using freezegun for these tests, I'd say we should use them also in other places where exact timing is important.