More comprehensive tests for trimming

[mxgrey]

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Description

We support an operation for "trimming" a workflow, meaning cancelling any activity going on within a specified span of the workflow. We have a rudimentary test for this operation, but it would be good to have more comprehensive tests, especially tests that ensure the span calculation is working correctly.

Tests to validate the span calculations could be done with unit tests inside of operate_trim.rs that manually build pseudo-workflow topologies by spawning entities with the right components, rather than spawning an entire proper workflow.