Expand memoized_unittest and clarify error message.

Specifically:
 - Improve style in memoized_unittest
 - Make test methods clearer
 - Make error message clearer if a non-hashable type is passed to a memoized function
 - Add unit test for this

The purpose of this change is just to clarify the code.

Review-Url: https://codereview.chromium.org/2073193003
Cr-Commit-Position: refs/heads/master@{#401337}
2 files changed