Update documentation on how to run config tests

[redshiftzero]

Description

We need to update the documentation on the config tests as it is no longer accurate (the root-level testinfra directory is gone entirely as of #3703):

https://docs.securedrop.org/en/latest/development/testing_configuration_tests.html

[conorsch]

Related to #3208. IMO, those two should be handled together. Happy to update the docs as part of the next sprint.

[redshiftzero]

Reading through #3208, it sounds like those are more extensive changes, so unless the deprecation of the entire Vagrantfile is imminent, we should keep the tickets separate