crontabberise drop_old_partitions; bug 1014128; attempt #2

This addresses bug 1014128. Again. (Previous attempt at rebasing was a horrible disaster. :unamused:) 

`?r` @peterbe and/or @rhelmer :revolving_hearts: 
Intention. 

Actually `eq_` is just a wrapper around `assert`. But the reader of the code will this way see what the purpose of the test is. Like, what's really important and intentional. I sometimes use `assert` as a way to visually pronounce what I know and take for granted. 

There's this principle in test driven development (which we're doing a hybrid of) that you write a test first that fails on an `eq_()` or an `ok_()` and then you go ahead and write the code that makes the test pass.