virttest.utils_test: Correct tests script dir for shared script in run_virt_sub_test()

Now we move all the shared test scripts to generic/tests/

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1399 from FengYang/run_virt_sub_test

virttest.utils_test: Correct tests script dir for shared script in run_v...