Fix the rpl_crash_safe, rpl_mixing_engines, and rpl_mts_crash_safe
files in extra/rpl_tests. They were broken by a previous patch.
Now, they all use $mtr_variables instead of @server_variables,
which improves speed since we can do string comparison without making
a round trip to the server. Moreover, the files do not rely as much
on undocumented side-effects of other include files as they used to
(in particular, rpl_mts_crash_safe and rpl_crash_safe don't rely on
rpl_mixing_engines setting $commands to the empty string).