[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.
Merge pull request #1118 from kitsilanosoftware/udp-client-test-check-ipv6

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