Playwright: Ensure that master key is the expected one

[BillCarsonFr]

Quick improvements to the playwright verification test:

• Ensure that the new session sees the correct cross signing key (protects against regression that would rotate the key on the new session)
• Check also that the backup decryption key is correctly gossiped after interactive verification (only rust crypto as on legacy it's a bit flaky because the setting page doesn't refresh when secret is received; would have to close and re-open or wait before opening first time)

---

This change is marked as an internal change (Task), so will not be included in the changelog.

[richvdh]

@BillCarsonFr are you still working on this? it seems to be bitrotting.

[t3chguy]

Closing due to being abandoned