Harden and re-enable DualMode socket tests (#80715)

Harden and re-enable DualMode socket tests (#80715)

- UDP tests are moved to a non-parallel collection, since there is no way to prevent parallel UDP tests from intercepting each other's packets. This only increases the execution time by ~2 seconds.
- TCP interferences are prevented by a utility PortBlocker, which creates and binds a "shadow" socket of the opposite address family to prevent sockets in parallel tests to bind to the same port.
- Accept and SendTo tests are refactored to use SocketTestHelperBase<T>, reducing duplicate code.