Convert unit test make_shared<nano::node> to system.add_node.

[clemahieu]

When changing the lifetime of io_context it ocurred that a lot of unit tests constructed nodes manually via make_shored.

This change converts many usages to system.add_node. Some usages like the networking tests are not upgraded as they're testing specific operations on how nodes connect which system::add_node assumes.

[dsiganos]

I think some of these tests manually create a node so that there are no automatic network connections between the nodes.

[dsiganos]

For example, the test case bootstrap_processor.lazy_hash_pruning intentionally creates the nano_node instance without using add_node so that there are no connections between the 2 nodes and then manually does a tcp_establish to connect the two nodes when the test wishes to connect them.

[dsiganos]

@clemahieu I made some fixes to the tests and committed, please check that you agree with what I am doing, before I carry on and do the rest

[clemahieu]

Yes these fixes look good. I thought I had avoided conversions in tests that needed a starting disconnected node, for instance the network.* tests, but I missed these bootstrap ones.

[dsiganos]

The principle idea behind this PR is invalid. The idea is to modify the bootstrap and rep_remove tests to system.add_node() instead of manually making their own node. But they manually make their own node because they want a disconnected node. I have made a lot of improvements to the bootstrap unit tests, including comments to stop this mistake from happening again (it has happened before too), but I will commit them as a new PR.