[#60058920] Performance test framework

An initial hack to compare the router's performance with it's raw backends.
Depends on [vegeta](https://github.com/tsenart/vegeta) and needs to be
refactored into a helper.
[#60058920] Compile hash of all Vegeta results

So that we can compile mean, max and 99th percentile, in addition to 95th
percentile.
[#60058920] Actual tests for perf regressions

To test the three areas that we've currently identified:

- Two healthy backends.
- Ditto, and one slow backend.
- Ditto, and one missing backend.

Implementation of Vegeta has been moved to a helper. Currently uses the
default rate of 50rps and duration of 10s. Tests take a total of 60s.
[#60058920] Refactor to expose all vegeta stats

Return the entire results hash rather than just pulling the `latencies`
tree. This will allow us to compare other values.

Stuffing the results into a new `res` hash because Long Variable Names.
[#60058920] Check quantity/codes of Vegeta results

Expect that we get the same quantity of requests against the Router and
backend during our timeboxed test, and that all of the responses have 200
status codes.
Merge pull request #48 from alphagov/60058920-performance_regression_tests

[Fixes #60058920] Tests for performance regressions