Consolidate testing docs

[infomiho]

Motivated by #2389 (comment)

We have multiple places where we talk about testing when using Wasp. We have our https://wasp-lang.dev/docs/project/testing docs where we talk about running client unit tests. And we also have e2e tests mention in the deployment docs.

We could probably talk about testing as one big topic and consider all the dimensions:

• (unit, e2e) tests
• (client, server) tests

And maybe also we can talk about what we recommend users for server testing.

Related to #2232 #2049