fix main.processlist_notembedded test
* before EXPLAINing `select sleep` wait for select to start
(fixes "Target is not running an EXPLAINable command")
* after killing sleep, wait for it to die
(fixes test failures on --repeat when old sleep shows on a test rerun)
* unify with 10.3, copy minor changes from there
(`--echo End of 5.5` vs `--echo # End of 5.5`, etc)
Do not blindly disconnect the connection that is in WAIT_FOR
because it could happen that neither the disconnect nor the
SIGNAL would be processed before RESET would discard the signal.
