Added option --vardir=<directory> for feature bug#10096


mysql-test/mysql-test-run.pl:
  Added option --vardir=<directory> for feature bug#10096
mysql-test/lib/mtr_process.pl:
  Added option --vardir=<directory> for feature bug#10096
mysql-test/lib/mtr_report.pl:
  Added option --vardir=<directory> for feature bug#10096