Task 40013: Write tests for data-wrangling parts in the visualize mode

I've added several tests to check whether the data frames produced by onionperf visualize contain the correct values when they are created. This patch also includes some small fixes for bugs caught whilst writing these. The scope of the tests includes filters operation, guards, errors, throughput, ttfb and ttlb values and numbers of downloads, for both 3.0 and 3.1 analysis file versions.

Having a test framework makes it easier to write tests for the new visualisations I produced as part of tpo/network-health/team#44 (moved)