"autosign" system test still fails occasionally due to apparent randomness issues

The autosign system test still fails occasionally due to the number of daily categories into which signature expiration times are divided is not high enough (4, which is less than the required minimum of 5):

I:autosign:checking expired signatures were jittered correctly (13)
I:autosign:404 20200121
I:autosign:202 20200123
I:autosign:599 20200124
I:autosign:405 20200125
I:autosign:error: not enough categories
I:autosign:failed

We should investigate our options here to prevent this test from failing as it is happening too often to just gloss over it. Assuming nothing is wrong with the implementation itself (or the test code checking the number of categories), we should either further lower the minimum required number of categories or ignore this issue completely - otherwise we are going to develop a bad reflex of ignoring all failures of this test.