Back-off retries for requests to `/3pid/onbind` against homeservers after a successful bind are not tested

[anoadragon453]

#552 fixed a bug where failed attempts to call PUT /_matrix/federation/v1/3pid/onbind by Sydent were not rescheduled properly (an exception was raised, and rescheduling the failed call thus never happened).

We should test that calls to /3pid/onbind are correctly retried in a back-off fashion if they fail.

Relevant code: https://github.com/matrix-org/sydent/blob/c09fe7af9484b358cf2374ba4ea0059956db127b/sydent/threepid/bind.py#L203-L219