The billing tests were flaky because some assertions ran before the
storage nodes finish their work.
A new helper function in testplanet has been added to allow to wait for
storage nodes endpoints to finish their work. This function now it's
used in the billing tests for avoiding their flakiness.
This commit closes the ticket:
https://storjlabs.atlassian.net/browse/SM-403
A part of fixing other billing tests flakiness.
Change-Id: Iacb750af435f515c04b1e1d3510a218d184c9abc
Add a test for checking that the billing:
* it doesn't include upload traffic
* it includes download traffic
Change-Id: I1655c15c1fad642f77dd210f2014b2586ae10104
