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

e2e: Implicitely include the handshake in all tests

Instead of having to write it manually everytime

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.