test(syncfiles): cover the identity on the xdcp subrequest

Assert that the xdcp subrequest names a username and that it is the arrayref
form, since the consumers index it as ->[0] and a bare string does not survive
that on the non-hierarchical path.

Also pin the two things this depends on: that xdsh still derives
DSH_FROM_USERID from the request username, and that updatenode still passes a
username on its own xdcp call, so the two callers cannot drift apart again.