Refactor SequentialTasks & SequentialRuns Tests

[EmmaMunley]

Changes

This PR refactors the tests: TestDAGExecutionQueueSequentialTasks and TestDAGExecutionQueueSequentialRuns to reduce the scope of the variables to individual test cases so that the test cases no longer influence each other.
/kind cleanup

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Release Notes

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[lbernick]

@EmmaMunley I think the explanation here is not quite right: the tests do currently pass in isolation and all at once, but the problem is that they influence each other by sharing state, modifying it, and resetting it at the end of the test. I would reword this commit message/PR description to explain that we're reducing the scope of these variables to individual test cases rather than all of them at once.

[lbernick]

/lgtm
thanks for updating the commit message; please update the PR description too

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