Consolidate test plans on `securedrop` wiki; phase out other GitHub wikis

We have three GitHub wikis:
https://github.com/freedomofpress/securedrop/wiki
https://github.com/freedomofpress/securedrop-client/wiki
https://github.com/freedomofpress/securedrop-workstation/wiki

They're mainly used for updating test plans. Historically, they were more active to store meeting notes and architecture docs, but these days, such documents mainly live in:

• The dev docs
• GitHub issues
• Discussions (also centralized on securedrop)
• FPF's internal wiki
• Assorted GDocs

I would propose:

• Move test plans to securedrop wiki
• Review other materials for freshness; preserve useful other docs in https://developers.securedrop.org/ or securedrop wiki
• Close securedrop-workstation and securedrop-client wikis

This is to reduce the number of places we have to look for things, or that we consider when creating new docs; generally, the dev docs seem like the best place for tips & architecture documentation.

Start by reviewing the three linked GitHub wikis and the developer documentation at developers.securedrop.org to inventory test plans and other materials. Identify useful documents that need preservation or relocation, then verify that test plans are consolidated on the securedrop wiki and that the securedrop-workstation and securedrop-client wikis can be closed.