Remove the part of the force-join test that checks the nick changed
Because we actually don’t change the nick, we ignore it, just like in the
case of a normal join (of an other resource, or an other channel on the same
Demonstrate issue #3423 with e2e tests
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.