Reassign path info to variable subtest_dirs during executing subtest.

Couple of tests call sub test but dont assign the correct path information to subtest_dirs.
e.g: type_specific.io-github-autotest-qemu.pci_hotplug.hotplug_block.blk_virtio_blk.with_reboot
This patch fixed the issue.

Signed-off-by: Lin Ma <lma@suse.com>