Improve Test Coverage of Registration Flow

[justjanne]

Currently, we’re only testing a few parts of the registration flow.

As we’ve changed some parts of that flow, especially the email verification flow in matrix-org/matrix-react-sdk#8654 and matrix-org/matrix-js-sdk#2390, it became obvious how little coverage of this brittle flow exists.

We should increase coverage and test multiple different flows to ensure the new user experience is not affected by bugs during registration. @t3chguy suggested writing cypress tests for registration with email verification using e.g., https://hub.docker.com/r/mailhog/mailhog/