Add negative test cases wherever possible

Add negative test cases wherever possible

Yes, repeating same test code body many times
to test each unprintable character below ' '
is not elegant, but it gets us to > 50%
of negative test cases, with room for improvement.

Yes, this seems ugly, but does the job.

Yes, I will have to wash my hands after committing this.

So here I am, with this patch, before you,
Dear Reviewer, so you don't have to make it.

Titan! to whose immortal eyes
         The sufferings of mortality,
         Seen in their sad reality,
Were not as things that gods despise;

(...)

Lord Byron, Prometheus

Change-Id: I48d7466ef6ca4143bf759d9b70ce60bdd347935c