Backport v2023 <- v2025dev2: Adapt pstore test to use systemd-pstore

After systemd has started to support pstore the test needs to be adapted to use this new functionality. Upon startup, systemd-pstore will monitor and extract the pstore contents and save it inside /var/lib/systemd/pstore/.

This also fixes the failure in the current test as /sys/fs/pstore: is not longer available.

Signed-off-by: Nithin M N MN.Nithin@in.bosch.com