[System] Fix DnsTest.GetHostByName() test.

[System] Fix DnsTest.GetHostByName() test.

	Test Case Failures:
	1) MonoTests.System.Net.DnsTest.GetHostByName : Should raise a SocketException (assuming that 'www.unlikely.novell.com' does not exist)
	  at MonoTests.System.Net.DnsTest.GetHostByName () [0x00089] in /Users/jon/Development/mono-HEAD/mono/mcs/class/System/Test/System.Net/DnsTest.cs:212
	  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
	  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00054] in /Users/jon/Development/mono-HEAD/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:230

As it happens, 'www.unlikely.novell.com' DOES exist:

	$ ping www.unlikely.novell.com
	PING www.unlikely.novell.com (199.101.28.20): 56 data bytes
	64 bytes from 199.101.28.20: icmp_seq=0 ttl=57 time=29.399 ms

Apparently it's due to the "www." prefix; remove "www." and the site
isn't found, as expected:

	$ ping unlikely.novell.com
	ping: cannot resolve unlikely.novell.com: Unknown host

Update DnsTest.noneExistingSite so that the DnsTest.GetHostByName()
test passes.