e2e_checks_vault: Increase the threshold for the vault check

A rapid look in worker logs shows that some cooking can take quite some time [1]. As the
current check randomly chooses directory (over the last 3 months) without any hints as
to its potential size (and thus the time it will take to cook), it's the only simple
implementation for now.

Jun 03 02:07:26 worker11 python3[463635]: [2021-06-03 02:07:26,061:
INFO/ForkPoolWorker-28] Task
swh.vault.cooking_tasks.SWHCookingTask[91bee640-b3de-4fa4-828a-fb08c86c3aea] succeeded
in 515.9954653847963s: None