Test flake: global_acceptance_depends_on_block_announcement

[kantai]

This tries to fix test flakiness in signer::v0::global_acceptance_depends_on_block_announcement.

I believe the issue was that poor timing in the CI could cause the checks at the end to work like this:

1. info_before is checked with stacks height N.
2. next_block_and() issues a bitcoin block B+1
3. Before processing B+1, the signers upload stacks block N+1 (because the test directives are all reset). This happens without a new proposal being sent out.
4. next_block_and() finishes successfully because N+1 advances the stacks tip
5. info_after is set at N+1, which has consensus hash for Bitcoin Block B (not B+1)
6. The search for the sister proposal fails because no new proposals are submitted for Block B.

This test corrects this by making step 2 wait for a new stacks block to be produced in B+1's tenure.

[hstove]

LGTM! Nice catch.

[obycode]

👏

[blockstack-devops]

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