Fix scheduler test

[Deepskyhunter]

ARTIQ Pull Request

Check-condition updated for test_pending_priority

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 "due_date" exist 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)

All Pull Requests

• Use correct spelling and grammar.
• Close/update issues.

Code Changes

• Run flake8 to check code style (follow PEP-8 style). flake8 has issues with parsing Migen/gateware code, ignore as necessary.
• Test your changes or have someone test them. Mention what was tested and how.
• Check, test, and update the unittests in /artiq/test/ or gateware simulations in /artiq/gateware/test

Documentation Changes

Git Logistics

• Split your contribution into logically separate changes (git rebase --interactive). Merge/squash/fixup commits that just fix or amend previous commits. Remove unintended changes & cleanup. See tutorial.
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Licensing

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ARTIQ files that do not contain a license header are copyrighted by M-Labs Limited and are licensed under LGPLv3+.

[sbourdeauducq]

Looks weird (especially the part about ignoring KeyError) and does not address this comment:
#1888 (comment)

Is the comment incorrect? Why?