virttest.utils_test: Init subtest_dir in run_virt_sub_test

Init subtest_dir to None otherwise, it will report the variable
referenced before assignment.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1717 from ypu/run_virt_sub_test

virttest.utils_test: Init subtest_dir in run_virt_sub_test