scons: only wrap message with positive value

In case we have small TTY, scons failed with wrong testwrap value. Fix
the issue.

Change-Id: I8ec1d55c6856c1e592a57a68067091b796ac84ae
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35596
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed