test_scheduler: remove the exact ordering assertion

[LEXUGE]

ARTIQ Pull Request

Description of Changes

Currently the exact ordering is compared in the pending_priorities test which is not necessary and is non-deterministic. This patch only asserts the middle priority experiment is ran before the high priority experiment scheduled in the future.

Related Issue

#1888
And it's also failing locally on my local nix develop

Type of Changes

	Type
✓	🐛 Bug fix

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[dnadlinger]

There are #1920 and #1976 as well, but they seem to have fizzled out. This is the minimal fix that, for the specific issue tested here, has the equivalent effect (for the wrong ordering, the test would just hang for many hours/timeout anyway). As it retains the original intention from when I wrote the (sloppy) existing test, I'd say we just merge this now. #1976 would still be good to have for more complex scenarios, but if that becomes ready to merge, we can just replace the implementation from this PR wholesale – no extra work.

[dnadlinger]

Merged in 530f67f.