From 0.14 (2017 Mar) until before 0.19 (2019 Nov), the height of the last
block pruned was returned, subject to a bug if there were blocks left unpruned
due to sharing files with later blocks.
In #15991, this was "fixed" to the current implementation, introducing a new
bug: now, it returns the first *unpruned* block.
Since the user provides the parameter as a block to include in pruning, it
makes more sense to fix the behaviour to match the documentation.
Nodes are restarted and reconnected as part of the test. Afterwards
`sync_blocks` is called immediately on the nodes. `sync_blocks`
first checks that all the included nodes have at least one
connection. Since adding a connection is usually happening in a
thread, sometimes nodes could run into this check before the
connection was fully established so that it would fail the entire
test.
This fix uses the `connect_nodes` helper to make the connection the
nodes. `connect_nodes` has a wait for the connection built into it.
