"Next steps will be to write a test suite for Arc, which is to validate the behavior of the run time and functions. Has anyone tried to make such a thing yet?"
Look at the *.t files in http://github.com/awwx/ar. They should also be good for a crazy simple test harness, and it's explicitly in an MIT license.
Such tests are essential for me, not just to ensure compatibility, but to ensure that when Arcueid gets ported to a new architecture everything still works. This is the big problem I'm having now as I try to get Arcueid to run properly on 32-bit architectures.
I plan to break up the test suites for Arcueid into two, one involving low-level tests that are run via GNU Check (which we have today), and more high-level tests run by some putative unit testing framework, perhaps your own or something derived from it, that should run on any Arc implementation. I may spin out the latter set of test suites into a separate project of its own.