MDEV-29755 post-merge for 10.6+

1) remove unnecessary restriction on changing mode;
2) adding lost wsrep_replicate_myisam_basic test.

mysql-test/suite/sys_vars/r/wsrep_replicate_myisam_basic.result

@@ -0,0 +1,15 @@
+#
+# wsrep_replicate_myisam
+#
+# save the initial value
+SET @wsrep_mode_saved = @@global.wsrep_mode;
+
+# scope and valid values
+SET @@global.wsrep_mode=REPLICATE_MYISAM;
+SELECT @@global.wsrep_mode;
+@@global.wsrep_mode
+REPLICATE_MYISAM
+
+# restore the initial value
+SET @@global.wsrep_mode = @wsrep_mode_saved;
+# End of test

mysql-test/suite/sys_vars/t/wsrep_replicate_myisam_basic.test

@@ -0,0 +1,19 @@
+--source include/have_wsrep.inc
+
+--echo #
+--echo # wsrep_replicate_myisam
+--echo #
+
+--echo # save the initial value
+SET @wsrep_mode_saved = @@global.wsrep_mode;
+
+--echo
+--echo # scope and valid values
+SET @@global.wsrep_mode=REPLICATE_MYISAM;
+SELECT @@global.wsrep_mode;
+
+--echo
+--echo # restore the initial value
+SET @@global.wsrep_mode = @wsrep_mode_saved;
+
+--echo # End of test

sql/wsrep_var.cc

@@ -985,12 +985,6 @@ bool wsrep_max_ws_size_update(sys_var *self, THD *thd, enum_var_type)
 bool wsrep_mode_check(sys_var *self, THD* thd, set_var* var)
 {
   ulonglong new_wsrep_mode= var->save_result.ulonglong_value;
-  if (new_wsrep_mode && !WSREP_ON)
-  {
-    my_message(ER_WRONG_ARGUMENTS, "wsrep_mode can't be set "
-               "if wsrep_on=OFF", MYF(0));
-    return true;
-  }
   ulonglong old_wsrep_mode= wsrep_mode;
   wsrep_mode= new_wsrep_mode;
   if (wsrep_check_mode(WSREP_MODE_REPLICATE_MYISAM) ||