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)