selftests: Add plan line and fix result line syntax

The TAP version 13 spec requires a "plan" line, which has been missing.
Since we always know how many tests we're going to run, emit the count on
the plan line. This also fixes the result lines to remove the "1.." prefix
which is against spec, and to mark skips with the correct "# SKIP" suffix.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>