MDEV-21462 main.processlist_notembedded fails to clean up

Replace the 30-second sleep in the test with proper
DEBUG_SYNC interlocking.

mysql-test/r/processlist_notembedded.result

 #
 # MDEV-20466: SHOW PROCESSLIST truncates query text on \0 bytes
 #
-SET DEBUG_SYNC= 'before_join_optimize SIGNAL in_sync';
+SET DEBUG_SYNC= 'before_join_optimize SIGNAL in_sync WAIT_FOR go';
 SET DEBUG_SYNC= 'now WAIT_FOR in_sync';
+SET DEBUG_SYNC= 'now SIGNAL go';
 SET DEBUG_SYNC = 'RESET';
 End of 5.5 tests

mysql-test/t/processlist_notembedded.test

 source include/have_debug.inc;
 source include/have_debug_sync.inc;
 source include/not_embedded.inc;
+source include/count_sessions.inc;
 
 --echo #
 --echo # MDEV-20466: SHOW PROCESSLIST truncates query text on \0 bytes
@@ -10,9 +11,9 @@ connect (con1,localhost,root,,);
 
 connection con1;
 
-let $q= `select CONCAT("SELECT user FROM mysql.user WHERE user ='some", CHAR(0), "' or sleep (30)")`;
+let $q= `select CONCAT("SELECT user sleep FROM mysql.user WHERE user ='some", CHAR(0), "'")`;
 
-SET DEBUG_SYNC= 'before_join_optimize SIGNAL in_sync';
+SET DEBUG_SYNC= 'before_join_optimize SIGNAL in_sync WAIT_FOR go';
 --disable_query_log
 --send_eval $q;
 --enable_query_log
@@ -27,8 +28,12 @@ let SEARCH_PATTERN=sleep;
 source include/search_pattern_in_file.inc;
 remove_file $MYSQLTEST_VARDIR/tmp//MDEV-20466.text;
 
+SET DEBUG_SYNC= 'now SIGNAL go';
+
 disconnect con1;
 
 SET DEBUG_SYNC = 'RESET';
 
+source include/wait_until_count_sessions.inc;
+
 --echo End of 5.5 tests