Use a non relative latency threshold for perf tests

The problem is that our test backends are ridiculously fast, and
therefore, 200% approximately zero is still approximately zero.

This changes the test to assert that the router doesn't add more than
20ms latency to the requests.
Output latency numbers from tests.

So that we can see what's going on on CI
Merge pull request #53 from alphagov/non_relative_latency_threshold

Use a non relative latency threshold for perf tests