selftests/binderfs: Fix harness API usage

The binderfs test mixed the full harness API and the selftest API.
Adjust to use only the harness API so that the harness API can switch
to using the selftest API internally in future patches.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>