[WIP] Remove NetworkThread from individual test cases #10111

This commit starts the NetworkThread automatically in the test framework, which means that the individual test cases don't need to start the thread after they've added the p2p connections.

It also updates the NodeConnCB.add_connection() method to wait for verack by default. This means the test writer only needs to call add_connection() in order to have a p2p connection ready for use.

This builds on #10109 . Only the 'Remove NetworkThread from individual test cases' commit needs to be reviewed

As discussed @sdaftuar - should be another easy review.

Closing this for now. I still think this is the right way to go, but I need to sequence the commits differently. NodeConn and NodeConnCB need to be initialized differently to avoid the timing issues that are happening on Travis.