[Fixes #60058920] Tests for performance 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.

--

Depends on alphagov/packager#13

For discussion:

- Are the bounds too tight? I've seen some failures so far.
- Should these tests run for longer?
- Should we mark some/all as optional?
- Are we missing anything?