Testing: Add disposition with SIP to fixture.

[lukasgraf]

This adds a disposition to the fixture which is in state disposition-state-disposed and has a SIP package, ready to be delivered.

This is necessary because transports (which will be implemented as part of #5167) need to access the underlying blob on the filesystem. The blob will only have a stable filesystem location once it's committed though. Because in our IntegrationTestCase we only emulate transaction.commit(), but never actually do a hard commit, this requires the blob to have already been created as part of the fixture to be actually committed and accessible.

Checkliste

• Aktualisierung Dokumentation vorhanden/nötig?

[njohner]

LGTM. Just a bit hard to judge whether we really need a disposition that has been disposed in the fixture or if using self.disposition and making the disposition-transition-dispose in the tests that need the SIP would be good enough would be enough.

[njohner]

Ok never mind my comment, I guess I did not properly read your PR comment... it's all clear why we need this!