autotest: Include only parent dir of the source in PS4

Previous expansion was assuming there's only one instance of "test/"
string in source's path but this is not really the case on phy
systems running in the CI. Instead of relaying on the string pattern
of the path, simply disect it into basename_dir/source. So e.g.:

/home/vagrant/spdk_repo/spdk/autobuild.sh -> spdk/autobuild.sh
/home/vagrant/spdk_repo/spdk/test/nvmf/common.sh -> nvmf/common.sh

Signed-off-by: Michal Berger <michalx.berger@intel.com>
Change-Id: I6f458566547dcaf5508263b2b19f4e1bc0de4f32
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8836


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>