factoring out more test code

[Martin Sebor <se...@roguewave.com>]

Hey Anton, attached is a patch that factors some more of the
boilerplate code out of the test function and into a helper
class template. I would like to move it to 21.strings.h but
I'm wondering if you could first take a look at it to see
if you think it will be readily usable in all or most of the
other tests or if I'm missing something than might make it
more difficult.

Thanks
Martin