RPN Calculator: two additional test cases so that a naïve implementation cannot pass the test suite

[lauramcastro]

Got a student whose implementation of this exercise was passing all tests, but was not actually using the result of operation.(stack) at any time. Instead, it was manually returning :ok and {:ok, "operation completed"}.

This was passing the test suite because only one operation was being used as test case for either calculate!/2, calculate/2 and calculate_verbose/2. Simply adding another test case that uses a different operation function should prevent such naïve implementation from passing all the tests.

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[glennj]

... was not actually using the result of operation.(stack) at any time. Instead, it was manually returning :ok and {:ok, "operation completed"}.

Test-driven development at its most pure! ;)