selftests: Add version file to kselftest installation dir

As titled, adding version file to kselftest installation dir, so the user
of the tarball can know which kernel version the tarball belongs to.

Link: https://lore.kernel.org/r/20250610221248.819519-1-1997cui@gmail.com


Signed-off-by: Tianyi Cui <1997cui@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>