Trivial comment change in an e2e test

Always send direct messages to the user’s bare JID

This has the following advantages:

- Works while all resources are offline (persistent channels)
- Helps with combating duplicate messages when sent to different
  resources, resources going offline, carbon-copying etc. etc.

As a side effect, this also makes the MUC PMs not be MUC PMs and
always be emitted from the server-wide JID of the sending user.

Fixes #3313.

Update the tests to work with oragono

Merge remote-tracking branch 'linkmauve/die-gc1.0-die'

And modify the tests to work, by adding the <x/> node in all join presences

Add a test to check that XEP-45 stable ID thing is respected

See https://xmpp.org/extensions/xep-0045.html#message

Split all the e2e tests in their own files

Some duplication is avoided, but most importantly: it’s much much cleaner,
easier to find, functions have been cleaned up (functools.partial usages are
now hidden), etc.