kselftest/arm64: Convert za-fork to use kselftest.h

Now that kselftest.h can be used with nolibc convert the za-fork test to
use it. We do still have to open code ksft_print_msg() but that's not the
end of the world. Some of the advantage comes from using printf() which we
could have been using already.

This does change the output when tests are skipped, bringing it in line
with the standard kselftest output by removing the test name - we move
from

    ok 0 skipped

to

    ok 1 # SKIP fork_test

The old output was not following KTAP format for skips, and the
numbering was not standard or consistent with the reported plan.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>