Faster room joins: Add happy path tests for device list tracking

[squahtx]

This change tests device list tracking for pre-existing and new members
in partial state rooms.

In particular, we test that:

• remote device lists are correctly cached or not cached
• local users are told about potential device list changes in /sync's
  device_lists.changed/left
• local users are told about potential device list changes in /keys/changes

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Best reviewed commit by commit.
The first half of the commits overlap with #473.

[squahtx]

Expected to fail until the last bullet in matrix-org/synapse#12993 (comment) is resolved.

[richvdh]

Now that matrix-org/synapse#13913 is merged, this should pass?

[richvdh]

(I kicked CI)

[squahtx]

Now that matrix-org/synapse#13913 is merged, this should pass?

Yes, that's right. And it looks like it does.