Show expected output in test-case error

It can get a bit confusing as to what test is looking for what result.
Add a message to the failure case to clear things up.  Looks like:

---
 raise mismatch_error
MismatchError: 1 != 2: Error at pos 1, expected [1, 2, 3, 1] and got [1, 1, 1, 1]
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TrivialFix

Change-Id: I40b57394b270f419b032301f490d4ba791c66396
Merge "Show expected output in test-case error"