tests/filesystem: increase /usr size

[jrusz]

The test started failing on 8.10 GA. It seems that something changed in the system repos and the size we had originally set was not enough.

This pull request includes:

• adequate testing for the new functionality or fixed issue
• adequate documentation informing people about the change such as
  • submit a PR for the READMEs listed here
  • submit a PR for the osbuild.org website repository if this PR changed any behavior not covered by the automatically updated READMEs

[achilleas-k]

Thank you!!

[achilleas-k]

Summarising some of our chat discussions for future context:
It's strange that something changed so dramatically in a GA (presumably in a z-stream update?) that it made the size of files in /usr big enough to not fit into the partition. It's possible the size was barely enough and the change wasn't really that big, but without looking at the old vs new to compare it's hard to say.

It's probably not worth spending time investigating this too deeply unless we see the same issue cropping up in other distro versions.