selftests/run_kselftest.sh: Resolve BASE_DIR with pwd -P

selftests/run_kselftest.sh: Resolve BASE_DIR with pwd -P

run_kselftest.sh only needs to canonicalize the directory containing the
script itself. Use shell-native path resolution for that by changing into
the directory and calling pwd -P.

This avoids depending on either realpath or readlink -f while still
producing a physical absolute path for BASE_DIR.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://lore.kernel.org/r/20260320-selftests-fixes-v1-3-79144f76be01@suse.com
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>