test: look up alerts sensor entity id dynamically

[seevee]

Context

The configured-name change made the hard-coded sensor.nws_alerts_alerts string literal unreliable in pytest.

EDIT: tests/test_sensor.py has asserted sensor.nws_alerts since a234bff (Aug 2024), and that slug has never matched a fresh install of this integration.

The proposed changes discover the entity via the sensor domain instead so the test survives future entity_id format changes.

Notes

• no-op change for working test setups
• fixes my own issues running tests against arbitrarily named entities
• statically configured test entity id -> now a dynamic entity lookup
• tests are now slightly more reliable for a wider range of entity ids, but still equally fragile to entity name scheme changes

[seevee]

Reopening with better justification than the original description. I framed this as preemptive hardening against 7eda048, but that attribution was wrong. The configured-name change isn't actually what makes the hard-coded slug unreliable.

The real situation: tests/test_sensor.py has asserted sensor.nws_alerts since a234bff (Aug 2024), and that slug has never matched a fresh install of this integration.

So the dynamic lookup here fixes a real, pre-existing bug in the test rather than a hypothetical future one, with zero user impact and no sensor.py changes.