Fixed random failure in main.kill_processlist-6619

wait for all previous connections to disconnect and for all previous
queries to finish running

mysql-test/main/kill_processlist-6619.test

@@ -4,8 +4,15 @@
 --source include/not_embedded.inc
 --source include/have_debug_sync.inc
 
+let $wait_condition=select count(*) = 1 from information_schema.processlist;
+source include/wait_condition.inc;
+
 --connect (con1,localhost,root,,)
 --let $con_id = `SELECT CONNECTION_ID()`
+
+let $wait_condition=select info is NULL from information_schema.processlist where id != $con_id;
+source include/wait_condition.inc;
+
 --replace_result Execute Query
 --replace_column 1 # 3 # 6 # 7 #
 SHOW PROCESSLIST;