There were several races in the main.kill_processlist-6619 testcase:

 - Lingering connections from a previous test case could be visible in SHOW
   PROCESSLIST and cause .result diff.
 - A sync point "dispatch_command_end" was ineffective, as it was consumed at
   the end of the SET DEBUG command itself.
 - The signal from sync point "before_execute_sql_command" could override an
   earlier signal, causing DEBUG_SYNC timeout and test failure.
 - The final SHOW PROCESSLIST could occasionally see a connection in state
   "Busy" instead of the expected "Sleep".
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>