Allow running flat tests

[PeterRugg]

(Pointed out by @nwf )
Sometimes you don't want a random generator, and you just want to be able to write out manual test cases. Currently, using the template mechanism to do this is inconvenient, as by default each test will be run 100 times, etc. Add a mechanism to support this use-case.

[nwf]

An added wrinkle: it may still be useful to shrink these flat tests, especially as I imagine using them to explore functional coverage cross products, and it'd be nice to have the "R"IG be able to help find the case or subsequence that goes wrong.