autotest: Add source and line number to PS4 prompt

Trace details are written out only when test fails, but sometimes
it's hard to determine from which script and what particular line
given command is being executed during the actual runtime of the
test. This should make it a bit easier. The log lines would look
like this:

-- common/autotest_common.sh@1217 -- # uname -s

Signed-off-by: Michal Berger <michalx.berger@intel.com>
Change-Id: Iac6973f986e6c553f39e5a9c909ccabf607251f7
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8756


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>