selftests: rds: Fix stale log clean up

Since rds self tests no longer has a default folder, users must
specify a log collection folder if they want to collect logs.
Currently the log folder is deleted and recreated, but this can
be dangerous if the user exports RDS_LOG_DIR=/tmp or /var/log.
This patch corrects the clean up to delete only rds log artifacts
from the log folder, and further prefixes rds specific logs as rds*

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260507233213.556182-2-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>