Fixed flaky SlaveTest.RegisteredAgentReregisterAfterFailover.
Fixed flaky SlaveTest.RegisteredAgentReregisterAfterFailover.

Previously due to a race condition a registration retry could be held
in the authorizer and forwarded back to the master only after the
agent's disconnection is detected, which leads to a remove registrar
operation when the test asserts there's none.

Review: https://reviews.apache.org/r/65438

Gilbert88/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,gsantovena/mesos,Gilbert88/mesos,andschwa/mesos,jpeach/mesos,jpeach/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,dforsyth/mesos,jpeach/mesos,dforsyth/mesos,andschwa/mesos,abudnik/mesos,gsantovena/mesos,kaysoky/mesos,jpeach/mesos,kaysoky/mesos,chhsia0/mesos,asamerh4/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,asamerh4/mesos,dforsyth/mesos,chhsia0/mesos,asamerh4/mesos,asamerh4/mesos,reneploetz/mesos,chhsia0/mesos,reneploetz/mesos,chhsia0/mesos,gsantovena/mesos,jpeach/mesos,kaysoky/mesos,abudnik/mesos,andschwa/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,reneploetz/mesos,andschwa/mesos,abudnik/mesos,Gilbert88/mesos,asamerh4/mesos,asamerh4/mesos,reneploetz/mesos,abudnik/mesos,andschwa/mesos,abudnik/mesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,gsantovena/mesos,gsantovena/mesos,chhsia0/mesos,asamerh4/mesos,abudnik/mesos,chhsia0/mesos,andschwa/mesos,kaysoky/mesos,gsantovena/mesos,Gilbert88/mesos,andschwa/mesos,jpeach/mesos,gsantovena/mesos,Gilbert88/mesos