Add default delay to fixture HTTP calls.

[smn]

Currently the test framework immediately returns results for http calls, that's not how things will happen in production.

Having a configurable delay in the fixtures would help, a default of 100ms would probably be OK?
It would help indicate where too many http calls are being made for production (even though they're snappy in the tests).

[hodgestar]

I don't want to add 1/10 or 2/10 of a second to lots of tests in order to
catch this. Maybe the dummy API should raise an error if too many HTTP
calls are made?