re-enable multiple test runs in sequence for 4.1 on non-windows

mysql-test/mysql-test-run.pl:
  delete $default_vardir if the 4.1 vardir trick is to be used to re-enable more than one test run in sequence

mysql-test/mysql-test-run.pl

@@ -820,6 +820,9 @@ sub command_line_setup () {
     {
       # Only platforms that have native symlinks can use the vardir trick
       $opt_mem= $opt_vardir;
+      # Delete the default directory in case it exists since we're going to create
+      # a new one later anyway, and the mkpath below throws a fatal error otherwise
+      eval { rmtree($default_vardir, 1, 1); };
       mtr_report("Using 4.1 vardir trick");
     }