Ensure SMP authentication test case works

[olabini]

Ensure that the test case in https://github.com/otrv4/pidgin-otrng/wiki/Test-Case---SMP-authentication works correctly. In the process of doing that, fill in the documentation in the wiki with more details where necessary.

[claucece]

Adding to this, the SMP authentication only shows up on the conversation window, so it is the only case where it should be tested. It also has two paths: 'shared secret' and 'question and answer'.

[MauroVelasco]

The first step:

When I log in, either prekey messages get published or a messages says that we have enough prekey messages.

Was not possible to verify, because the XMPP server is not showing any message related to "We have enough prekey messages"