improve wk-lib snapshot tests

[jstriebel]

Currently, some points should be addressed for the wk-lib snapshot tests:

• Some requests are run against webknossos.org, which might leak user data. For this reason some snapshots are edited manually, which makes it hard to upgrade them.
• The snapshots contain many timestamps, which makes them change, so that refreshing the snapshots creates large diffs. We should filter those timestamps so that a refresh only shows the necessary changes. There are hooks for this in vcrpy
• The current version of the local webknossos setup is a bit hidden in the test.sh, maybe we could default to the currently running webknossos.org version?
• The database should be dropped or evolutions must be run when upgrading the local webknossos setup.
• The tests should be run regularly without snapshots, e.g. as a nightly test, so that we get notified when webknossos.org has breaking changes we didn't anticipate.
• __generate_client.py needs some cleanup:
  • use isolated wk
  • use extra module for responses
  • use test data
  • integrate into wk? see also Find a way to include response schemas in swagger/openAPI webknossos#6017

[jstriebel]

pointers for myself:

• https://github.com/scalableminds/webknossos/wiki/Set-up-a-standalone-datastore-locally
• https://github.com/scalableminds/webknossos/blob/master/tools/postgres/prepareTestDB.sh

[jstriebel]

Also, __generate_client.py needs some cleanup:

• use isolated wk
• use extra module for responses
• use test data
• integrate into wk?