Fix testresample.c output span; add exit code

Prior to this chance, the "Resample with different factors" test only passed for 60 of the 63 factors, with the 3 failing ones being the largest. 1. Since only 63 distinct factors were being considered, 100 random samples was overkill. 2. To support noticing failure in continuous build systems, it's nice if the test exit()s with nonzero when there are failures. 3. The root cause was a formula error when determining which indices in the resampled output ought be compared. Details are explained in a comment.