Add Must[Get|Set]DisplayName and use MustUploadKeys in more places

[kegsay]

Small refactor to add a few more helpers. The hitlist for this can be discovered using grep --only-matching -Er '\[\]string\{\"_matrix\".*\}' tests | grep -E --only-matching '\[\]string.*' | sort | uniq -c | sort which currently spits out:

...
   4 []string{"_matrix", "client", "v3", "account", "whoami"}
   4 []string{"_matrix", "client", "v3", "devices"}
   4 []string{"_matrix", "client", "v3", "directory", "room", aliasName}
   4 []string{"_matrix", "client", "v3", "logout"}
   4 []string{"_matrix", "client", "v3", "rooms", roomID, "send", "m.room.message", "txn-1"}, client.WithJSONBody(t, map[string]interface{}
   4 []string{"_matrix", "client", "v3", "rooms", roomID, "state", "m.room.topic"}
   5 []string{"_matrix", "client", "v3", "rooms", roomID, "kick"}
   5 []string{"_matrix", "client", "v3", "rooms", roomID, "state", "m.room.member", bob.UserID}
   5 []string{"_matrix", "client", "v3", "rooms", roomID, "state", "m.room.power_levels"}
   6 []string{"_matrix", "client", "v3", "keys", "claim"}
   6 []string{"_matrix", "client", "v3", "keys", "device_signing", "upload"}
   6 []string{"_matrix", "client", "v3", "login"}, client.WithJSONBody(t, map[string]interface{}
   6 []string{"_matrix", "client", "v3", "rooms", roomID, "members"}
   6 []string{"_matrix", "client", "v3", "rooms", roomID, "state", "m.room.name"}
   6 []string{"_matrix", "client", "v3", "sync"}
   7 []string{"_matrix", "client", "v1", "rooms", roomID, "relations", rootEventID}
   7 []string{"_matrix", "client", "v3", "login"}
   7 []string{"_matrix", "client", "v3", "presence", alice.UserID, "status"}
   7 []string{"_matrix", "client", "v3", "rooms", roomID, "forget"}, client.WithJSONBody(t, struct{}{}
   7 []string{"_matrix", "client", "v3", "search"}
   7 []string{"_matrix", "client", "v3", "user_directory", "search"}
   8 []string{"_matrix", "client", "v3", "devices", newDeviceID}
   9 []string{"_matrix", "client", "v3", "keys", "query"}
   9 []string{"_matrix", "client", "v3", "register"}
  10 []string{"_matrix", "client", "v1", "rooms", root, "hierarchy"}
  10 []string{"_matrix", "client", "v3", "keys", "upload"}
  18 []string{"_matrix", "client", "v3", "rooms", roomID, "messages"}

The point of helpers in the CSAPI struct is:

• to provide a shortcut when the test isn't really testing that endpoint (so it just needs it as boring setup code)
• it's used multiple times by different tests

Originally we didn't have enough tests to meet that 2nd bullet point but this hasn't been true for quite some time, but we never revisited adding helpers until now.