E2E testing - Figure out expected_output

[eyal-mor]

Original Author: @dkirillov

We should figure out how the E2E testing behaves and the interface for it - specifically around expected_output and the various stage output cases.

Some things that come up are:

• How strict we are with tests.
• If the stage produces nothing, do tests need to check for nothing.
• If a stage produces something, but we don't care, do we still need to check for it.
• If a stage produces something, but that something is dynamic between each run, how do we check for that (or do we).