Document E2E helpers, best practice etc

[dtrudg]

Type of issue

Developer Documentation

Description of issue

The e2e suite is quite complicated, with a lot of helper functions, and established patterns for performing certain types of tests. These are not well documented, if at all.

We should have sufficient documentation and examples that a mid-level Go developer can read the e2e developer documentation and then be able to understand add new tests, or modify tests.