After syncing the blocks, we didn't check that the
indexes have caught up to the tip before manually pruning.
This could lead to prune blockers lower thatn the expected height.
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.
