Testing guide is out of date

[flexdinesh]

In #7968 we removed the exports from @keystone-6/core/testing. Testing guide still has examples of non-existing exports from @keystone-6/core/testing. Guide needs to be updated.

[sharmsa66]

@flexdinesh , Thanks for your response then please let us know how one should test context API with something like Postgres, do we need to have Postgres running? it will be great if you can provide an example for testing the express app also.

[rburgst]

Please also note #8324

[Thinkscape]

The docs still describe non-existent classes:

I think we should completely drop this section: https://keystonejs.com/docs/guides/testing#test-runner

Unsure if setupTestEnv is coming back; Otherwise we should remove this: https://keystonejs.com/docs/guides/testing#test-environment

[dcousens]

@Thinkscape I don't think setupTestEnv is coming back, getContext is 99.99% of the utility without the issues.
Could you open a pull request to remove the problematic pages? 💛