MDEV-19904 poradic buildbot failures in rpl.rpl_semi_sync_uninstall_plugin

The test allowed non-deterministic execution thanks to unresetable status
var of Slave_connections.

Fixed with expecting a correct value for Slaves_connected.

mysql-test/suite/rpl/r/rpl_semi_sync_uninstall_plugin.result

@@ -25,10 +25,6 @@ select plugin_name,plugin_status from information_schema.plugins where plugin_na
 plugin_name	plugin_status
 rpl_semi_sync_slave	DELETED
 [connection master]
-show global status like "Slave%_connect%";
-Variable_name	Value
-Slave_connections	2
-Slaves_connected	1
 UNINSTALL PLUGIN rpl_semi_sync_master;
 Warnings:
 Warning	1620	Plugin is busy and will be uninstalled on shutdown

mysql-test/suite/rpl/t/rpl_semi_sync_uninstall_plugin.test

@@ -68,8 +68,13 @@ select plugin_name,plugin_status from information_schema.plugins where plugin_na
 --echo [connection master]
 
 # The following is to catch errors if the next uninstall plugin would succeed
-show global status like "Slave%_connect%";
-
+let $slaves_connected_value= query_get_value(SHOW GLOBAL STATUS LIKE 'Slaves_connected', Value, 1);
+if (`SELECT $slaves_connected_value <> 1`)
+{
+  SHOW GLOBAL STATUS LIKE 'Slave_connections';
+  SHOW GLOBAL STATUS LIKE 'Slaves_connected';
+  --die *** Unexpected number of connected slaves ***
+}
 UNINSTALL PLUGIN rpl_semi_sync_master;
 select plugin_name,plugin_status from information_schema.plugins where plugin_name like 'rpl_%';