add assertAmountsEqual() as a testHelper

[Chris-Hibbert]

What is the Problem Being Solved?

t.truthy(math.isEqual(amount1, amount2)) provides no helpful debugging info. The replacement I've been using: t.deepEquals(amount1, amount2) works for natMath amounts, but is a bad habit because it would fail for sets.

Description of the Design

Add a helper (in zoeTestHelpers) that just compares the brands and values, and prints something helpful on failure.

Also, use the new helper in assertPayoutAmount(), etc., which currently use t.deepEqual().

Security Considerations

more reliable test results.

Test Plan

replace some uses in tests. Observe no change in test results.