Test suite: allow the pattern to cover less of the write cache

Same reasoning as on 87436e91: on
tails/tails!1518 I've seen "85.162% of the test FS size (127.599609375 MiB) has
the pattern, but more than 86.000% was expected".

And @anonym reported on
tails/tails!1640 (comment 241261) "On
jenkins I see six failures due to coverage being lower than 85%, the lowest
being 83.876%, so IMHO let's lower this to 80% or even 75%".