Add a test case for the signer race condition fixed in #4738

[obycode]

I think that we want to make sure in a realistic testing environment that signers are able to sign new blocks during the 0th tenure of a reward cycle. If there's a race condition in the /v2/pox endpoint which prevents it from updating on time, its possible it could trigger "off by one" like behavior for the 0th tenure. I think this patch fixes that, but its something we want to be sure to confirm.

Originally posted by @kantai in #4738 (review)