test: refactor NetworkingTests to take much less time.

[benhillis]

While looking at adding some additional tests for VirtioProxy networking mode, I noticed a lot of problems with the existing networking test class. This change is some minor updates that will:

1. Reduce the number of VMs that need to be created, this will improve runtime substantially
2. Improve the handling of WSL1 tests so the entire class is skipped instead of each test method individually