lisa.regression: Add Regression.fix_validation_min_iter_nr

Compute the number of iterations required to observe the effects of a
fix to a test case, compared to the new failing rate. This gives an
indication on how many iterations are needed to have `exekall compare`
answer the question "is my fix fixing this regression ?", assuming that
you actually fixed it. If `exekall compare` says that there is no
significant change after that number of iterations, chances are that the
fix is only partial (or not fixing anything at all).