[tests] make pruning test faster

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.

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