selftests/amd-pstate: Fix broken paths to run workloads in amd-pstate-ut

In selftests/amd-pstate, tbench and gitsource microbenchmarks are
used to compare the performance with different governors. In current
implementation the relative path to run `amd_pstate_tracer.py` is broken.
Fix this by using absolute paths.

Signed-off-by: Swapnil Sapkal <swapnil.sapkal@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>