Test suite: fix regression on Stretch introduced earlier on this branch

Fixup against 4efe5283.

Interestingly, I did not have this problem with Ruby 2.7.1 on my system,
but on Jenkins (Ruby 2.3.3) I see this error:

  wrong argument type String (expected Symbol) (TypeError)
  ./features/step_definitions/pidgin.rb:113:in `/^my XMPP friend goes online( and joins the multi-user chat)?$/'

Until we require Buster for running our test suite, the current implementation
is rather ugly, but I figured it's better to have an ugly backwards
compatibility workaround than to stick with old arguments passing style: it
brings us one step closer to Ruby best practices.