Add support for evaluating reducers in marble tests

[todo]

nteract/packages/myths/__tests__/myths.spec.ts

Lines 80 to 85 in 511999d

	// TODO: figure out how to add ability to evaluate reducers in a
	// marble test
	// Then the above should be:
	// B: addToSum.create(-4)
	// D: addToSum.create(1)
	// no emission on E

---

This issue was generated by todo based on a TODO comment in 511999d when #5106 was merged. cc @CrystallineCat.

[stale]

This issue hasn't had any activity on it in the last 90 days. Unfortunately we don't get around to dealing with every issue that is opened. Instead of leaving issues open we're seeking to be transparent by closing issues that aren't being prioritized. If no other activity happens on this issue in one week, it will be closed.
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