Test purge-keys option

Test purge-keys option

Add some more zones to the kasp system test to test the 'purge-keys'
option. Three zones test that the predecessor key files are removed
after the purge keys interval, one test checks that the key files
are retained if 'purge-keys' is disabled. For that, we change the
times to 90 days in the past (the default value for 'purge-keys').

(cherry picked from commit 6333ff15f0356878a34fe7c15d2a7cf9d24cd2d7)