selftests/run_kselftest.sh: Use readlink if realpath is not available

'realpath' is not always available,  fallback to 'readlink -f' if is not
available. They seem to work equally well in this context.

Link: https://lore.kernel.org/r/20250318160510.3441646-1-yosry.ahmed@linux.dev


Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>