lastfailed - persist failure sets until all broken tests where removed

[RonnyPfannschmidt]

it occurred to me that while doing red-green refactoring,
the likelihood of breaking tests in the same failure set is high,
it might be helpful to have an option to keep them around

i propose adding a --lf-keep option that would simply skip writing the new last-failed if there are leftover failures

[RonnyPfannschmidt]

followup to #1128 - when keeping the current failure set in a test subset selection
its not clear when to discard a part of the failed set