Avoid mtr warning on Windows during startup

The reason for mtr warning is that collect_mysqld_features() starts mysqld with --datadir=/tmp and this 
directory does not exist on Windows.

Fix : instead of passing --datadir=$opt_vardir/tmp in collect_mysqld_features() just use --datadir=.
mysqld does not need a correct directory, just an existing one, as it is started with --help ---verbose -skip-grant-tables.

mysql-test/mysql-test-run.pl

@@ -1697,7 +1697,7 @@ sub collect_mysqld_features {
   my $args;
   mtr_init_args(\$args);
   mtr_add_arg($args, "--no-defaults");
-  mtr_add_arg($args, "--datadir=%s/tmp", $opt_vardir);
+  mtr_add_arg($args, "--datadir=.");
   mtr_add_arg($args, "--basedir=%s", $basedir);
   mtr_add_arg($args, "--language=%s", $path_language);
   mtr_add_arg($args, "--skip-grant-tables");