Add zebrad acceptance tests for the mempool

[teor2345]

Motivation

The mempool unit tests cover a lot of mempool functionality, using MockServices where needed.

But there are some things we can't test unless we actually run zebrad:

• mempool startup from zebrad's start command
  • the unit tests already check that the mempool starts disabled
• mempool startup while the rest of zebrad is running (covered by PR Add a debug config that enables the mempool #2862)
• mempool running while the rest of zebrad is also running
• The mempool doesn't activate really early (while we're syncing the first few checkpoints)

We've had panics and deadlocks in similar situations before. (Particularly the start command & running everything at the same time.)

So it's important that we run some tests like this.

[mpguerra]

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[oxarbitrage]

Related: #2690

[teor2345]

Unblocked by merging PR #2862.