Make absence of min-file indicates first run

Consider an n-way differential testing. In the first run, we notice that P_n-1 and P_n disagree, so there is no need to invoke P_1, ..., P_n-2 any further for newer programs.

One possible strategy is that the runner will write information which backends disagree, and subsequent invocations will read this information. This requires the runner to recognize when it is the first invocation (to determine whether it should read or write the information). One unintrusive way to do this is to check for the absence of last-good-min-file