Run an unbounded number of splits during passthrough

The python test fixture PortAllocator only allows each fixture to reserve up to twenty ports for its servers. Each time we want to split a donor we need to add three new nodes, which quickly drains the available port range given to us. SERVER-65042 covered allowing multiple (more than one) shard splits to happen during passthrough testing, and this ticket will cover running an unbounded number of splits during that time. Two approaches that might work:

• Stop tracking more than one retired donor, or any retired donors, and reuse the ports of existing replica sets after they are retired
• Implement a new PortAllocator that let's us use more than 20 ports per fixture run