MDEV-7294 MTR does not use /dev/shm with a out-of-source build

mtr internally does the following:
1. $default_vardir=....
2. $opt_vardir=$default_vardir unless $opt_vardir;
3. $opt_vardir=realpath $opt_vardir unless IS_WINDOWS
4. if ($opt_vardir eq $default_vardir) { .... use /dev/shm ... }

thus we have to realpath $default_datadir too, otherwise
the comparison logic might be broken

mysql-test/mysql-test-run.pl

@@ -1494,6 +1494,7 @@ sub command_line_setup {
   {
     $default_vardir= "$glob_mysql_test_dir/var";
   }
+  $default_vardir = realpath $default_vardir unless IS_WINDOWS;
 
   if ( ! $opt_vardir )
   {