Fixed failure in flush_read_lock.test

The failure was:
mysqltest: At line 1737: query 'reap' failed: 1397: XAER_NOTA: Unknown XID

The bug was in the test case int that it executed REAP XA COMMIT before
the connection had truly disconnected

mysql-test/main/flush_read_lock.result

@@ -1391,9 +1391,11 @@ insert into t3_trans values (2);
 xa end 'test2';
 xa prepare 'test2';
 # Disconnect temporary connection
+set debug_sync='thread_end SIGNAL detached';
 disconnect con_tmp;
 # Switching to connection 'con1'.
 connection con1;
+set debug_sync='now WAIT_FOR detached';
 flush tables with read lock;
 # Switching to connection 'default'.
 connection default;

mysql-test/main/flush_read_lock.test

@@ -1698,9 +1698,11 @@ insert into t3_trans values (2);
 xa end 'test2';
 xa prepare 'test2';
 --echo # Disconnect temporary connection
+set debug_sync='thread_end SIGNAL detached';
 disconnect con_tmp;
 --echo # Switching to connection '$con_aux1'.
 connection $con_aux1;
+set debug_sync='now WAIT_FOR detached';
 flush tables with read lock;
 --echo # Switching to connection 'default'.
 connection default;