selftests/run_kselftest.sh: Allow choosing per-test log directory

selftests/run_kselftest.sh: Allow choosing per-test log directory

The --per-test-log option currently hard-codes /tmp. However, the system
under test will most likely have tmpfs mounted there. Since it's not clear
which filenames the log files will have, the user should be able to specify
a persistent directory to store the logs. Keeping those logs are important
because the run_kselftest.sh runner will only yield KTAP output, trimming
information that is otherwise available through running individual tests
directly.

Allow --per-test-log to take an optional directory argument. Keep the
existing behaviour when the option is passed without an argument, but if
a directory is provided, create it if needed, reject non-directory paths
and non-writable directories, canonicalize it, and have runner.sh write
per-test logs there instead of /tmp.

This also makes relative paths safe by resolving them before the runner
changes into a collection directory.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://lore.kernel.org/r/20260320-selftests-fixes-v1-4-79144f76be01@suse.com
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>