add back workaround for account deactivation test.

This workaround this issue: when starting this test after the two others, ACME staging server seems to have again in cache some TXT record and return error to get certificate.
As this test just use acme-dns-tiny to register the account key, we just check the registration is working.
3 jobs for v2 in 17 minutes and 26 seconds (queued for 1 minute and 12 seconds)
Status Job ID Name Coverage
  Test
passed #309
jessie

00:06:19

90.0%
passed #310
jessie_backport

00:05:36

90.0%
passed #311
stretch

00:05:30

90.0%