Also use cosntant backoff retry strategy as opposed to exponential
backoff. This is in case retrying several times quickly causes the
retry intervals to become large enough that the test is very slow
and/or the Bitcoin lock transaction expires.
The current problem occurs on the last message i.e. Bob sending
tx_redeem_encsig to Alice. The action is yielded for Bob to do it, but
Alice appears to never receive it (unconfirmed claim, requires more
logging).
