selftests: kselftest_harness: separate diagnostic message with # in ksft_test_result_code()

According to the spec we should always print a # if we add
a diagnostic message. Having the caller pass in the new line
as part of diagnostic message makes handling this a bit
counter-intuitive, so append the new line in the helper.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>