MDEV-7352: main.kill_processlist-6619 fails sporadically in buildbot

After fix of MDEV-6728, the KILL signal is reset at the start of query
execution. This means that in this test case, we need to wait for the
to-be-killed query to have started; otherwise the kill signal can be lost,
causing the test case to fail.

mysql-test/r/kill_processlist-6619.result

@@ -3,13 +3,15 @@ SHOW PROCESSLIST;
 Id	User	Host	db	Command	Time	State	Info	Progress
 #	root	#	test	Sleep	#	#	NULL	0.000
 #	root	#	test	Query	#	#	SHOW PROCESSLIST	0.000
-SET DEBUG_SYNC='before_execute_sql_command WAIT_FOR go';
+SET DEBUG_SYNC='before_execute_sql_command SIGNAL ready WAIT_FOR go';
 SHOW PROCESSLIST;
 connection default;
+SET DEBUG_SYNC='now WAIT_FOR ready';
 KILL QUERY con_id;
 SET DEBUG_SYNC='now SIGNAL go';
 connection con1;
 ERROR 70100: Query execution was interrupted
+SET DEBUG_SYNC='reset';
 SHOW PROCESSLIST;
 Id	User	Host	db	Command	Time	State	Info	Progress
 #	root	#	test	Sleep	#	#	NULL	0.000

mysql-test/t/kill_processlist-6619.test

@@ -9,14 +9,19 @@
 --let $con_id = `SELECT CONNECTION_ID()`
 --replace_column 1 # 3 # 6 # 7 #
 SHOW PROCESSLIST;
-SET DEBUG_SYNC='before_execute_sql_command WAIT_FOR go';
+SET DEBUG_SYNC='before_execute_sql_command SIGNAL ready WAIT_FOR go';
 send SHOW PROCESSLIST;
 --connection default
+# We must wait for the SHOW PROCESSLIST query to have started before sending
+# the kill. Otherwise, the KILL may be lost since it is reset at the start of
+# query execution.
+SET DEBUG_SYNC='now WAIT_FOR ready';
 --replace_result $con_id con_id
 eval KILL QUERY $con_id;
 SET DEBUG_SYNC='now SIGNAL go';
 --connection con1
 --error ER_QUERY_INTERRUPTED
 reap;
+SET DEBUG_SYNC='reset';
 --replace_column 1 # 3 # 6 # 7 #
 SHOW PROCESSLIST;