Test cases for flow analysis

[krame505]

There don't appear to be any test cases for the flow analysis, besides just running it for ableC and other projects - but we should definitely be checking for regressions in various flow errors being reported.

I thought that I recalled there being a flow/ directory under tests/ at one point, but this doesn't seem to be the case. Were these tests accidentally deleted at one point, or where they never there in the first place? Do @tedinski or @ericvanwyk remember?

In the meantime I may start adding some tests dealing with references.