(hotfix) modernize a really old test
(hotfix) modernize a really old test

This test was written in the legacy Test::Unit style format over
7 years ago.  It was added in PR 80.  I noticed this while working
on the with backtrace context tests recently.

This switches to the modern assert `should "..." do` implementation
in prep for eventually removing the legacy Test::Unit syntax.

redding/assert