Add mock to the list of whisper's test inputs.

This was required as the versiof whisper was upgraded from
1.1.3 to 1.1.4 and mock was added as a test dependency in
that release:
https://github.com/graphite-project/whisper/commit/703dd9d3a60a0f79383c03a541ee66bd50966a01#diff-62849af500c571aa4532da7be8833e73R14