Fix scheduler test

[Deepskyhunter]

…ng_priority

ARTIQ Pull Request

Description of Changes

Goal of the test: Check due dates take precedence over priorities when waiting to prepare.
Problem: Irrelevant case cannot pass the test
Solution: Check is "value" a dictionary before assertEqual() to narrow down the cases needed to be compared.

Related Issue #1888

Type of Changes

	Type
✓	🐛 Bug fix

Steps (Choose relevant, delete irrelevant before submitting)

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• Check, test, and update the unittests in /artiq/test/ or gateware simulations in /artiq/gateware/test

Documentation Changes

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[Deepskyhunter]

I misunderstood the meaning of the item inside expect.