Need to add test artifacts to .gitignore

[Dan Jacques]

FYI, I've noticed when building from "pu" that neither the
"t/helper/test-print-values" or "t/helper/test-print-larger-than-ssize"
testing artifacts added to Makefile in this patch series are not added to
"t/helper/.gitignore" like other helpers, resulting in the testing artifact
being recognized as an untracked file.