This commit makes the pruning.py much faster. Key insights to do this: - pruning.py doesn't care what kind of transactions make up the big blocks that are pruned in the test. Instead of making blocks with several large, expensive to construct and validate transactions, instead make the large blocks contain a single coinbase transaction with a huge OP_RETURN txout. - avoid stop-starting nodes where possible. This test could probably be made even faster by using the P2P interface for submitting blocks instead of the submitblock RPC.
Completes backport of core PR15686 (commit 03d6d23):
Depends on D4561.