MDEV-27738 Windows : mysql-test-run --extern does not work

Use portable quoting in mtr_add_arg.

mysql-test/mysql-test-run.pl

@@ -1908,7 +1908,7 @@ ()
   }
 
   mtr_add_arg($args, "--silent"); # Tab separated output
-  mtr_add_arg($args, "-e '%s'", "use mysql; SHOW VARIABLES");
+  mtr_add_arg($args, "-e \"use mysql; SHOW VARIABLES\"");
   my $cmd= "$mysql " . join(' ', @$args);
   mtr_verbose("cmd: $cmd");