Add FAQ entry for tests scenarios that might be running slow

[manusa]

Description

Users who are testing for failure scenarios (5xx, 4xx responses) might be running tests at slower speeds due to the automatic retry policy the client has.

We need to add a FAQ entry to make this clearer so that users are able to properly configure the client when running these kind of tests.

The content for the FAQ entry should be similar to:

#5422 (comment)

Relates to #6560