Define "test page" for verifying Erika implementation against ErikaMock implementations

[ArchibaldBienetre]

We had some issues in the past that the test double implementations diverged from the implemented hardware interface (for various reasons, e.g. choice of level of abstraction).

If we design a test page that can be compared with on-screen output and output on hardware ("this box should exactly contain 'hello world'") - or even just a specific hardware-page that contains all the instructions.