Add diff tests to lambkin.shepherd

[hidmic]

Feature description

In order to use benchmarks as performance regression tests, we need a way to assert on a performance regression. To do that, we need a way to summarize benchmark results, compare these benchmark summaries, and fail a benchmark when the comparison suggests a problem.

Implementation considerations

Summaries should be user-defined if at all possible (it really depends on the application which metrics matter and which don't). This ticket will likely require extending RobotFramework libraries in lambkin.shepherd.