[System.Net] Do not assume IPv6 is enabled in UdpClientTest

Without this, three of the tests fail on Linux kernels without IPv6
support.

This is a minimal fix, copy'n'pasting a conditional which was used in
other tests.  It might be nicer to factor out the `SupportsIPv6` to its
own utility function.

Let me know if you'd like me to take care of that small refactoring,
possibly in a second patch (as to not obscure the intent).