MDEV-26527 speedup appveyor build

Allow mysql_client_test to execute even if server/client auth plugins are
not built. There is a single small test that requires qa_auth_client.

mysql-test/main/mysql_client_test.test

 # This test should work in embedded server after we fix mysqltest
 -- source include/not_embedded.inc
-# need to have the dynamic loading turned on for the client plugin tests
---source include/have_plugin_auth.inc
 
 # Run test with default character set
 --source include/default_charset.inc

tests/mysql_client_test.c

@@ -19203,6 +19203,9 @@ static void test_bug11766854()
   struct st_mysql_client_plugin *plugin;
 
   DBUG_ENTER("test_bug11766854");
+  if (!getenv("QA_AUTH_CLIENT_SO"))
+    DBUG_VOID_RETURN;
+
   myheader("test_bug11766854");
 
   plugin= mysql_load_plugin(mysql, "foo", -1, 0);