Begin porting scanimage tests to C++
I'd like to start making some improvements to the scanimage
frontend. Before this, I'd like to build up a suite of regression tests. In order to do this, I moved minigtest into a common location, then started porting the tests that live in testsuite/Makefile to C++.
I'm creating this pull request to show what direction I'm headed and to get feedback (as well as to see if it passes CI!).