Flaky unit test in blocks_sensing.js

[kchadha]

Expected Behavior

Tests should not be flaky.

Actual Behavior

The unit test for the loudness block is flaky, particularly here:
https://github.com/LLK/scratch-vm/blob/develop/test/unit/blocks_sensing.js#L198-L202

Here is the raw log from a travis build:
flaky-unit-test-vm.txt