[System] Use common assertion api in IPAdressTest to fix monodroid build

`Assertion.Fail` isn't supported by NUnitLite in monodroid, use `Assert` instead to fix the build:

```
[12:29:46] /Users/builder/data/lanes/1845/88dbce4e/source/monodroid/tests/bcl-test/BclTests.csproj (default targets) ->
[12:29:46] /Library/Frameworks/Mono.framework/Versions/4.2.1/lib/mono/4.5/Microsoft.CSharp.Targets (CoreCompile target) ->
[12:29:46] 
[12:29:46] 	src/System/Test/System.Net/IPAddressTest.cs(282,4): error CS0103: The name `Assertion' does not exist in the current context
[12:29:46] 
[12:29:46] 	 14 Warning(s)
[12:29:46] 	 1 Error(s)
```

mcs/class/System/Test/System.Net/IPAddressTest.cs

@@ -279,7 +279,7 @@ public class IPAddressTest
 	{
 		try {
 			IPAddress ip1 = new IPAddress (0x0000000100000000);
-			Assertion.Fail ("#1");
+			Assert.Fail ("#1");
 		} catch (ArgumentOutOfRangeException) {}
 		
 		IPAddress ip = IPAddress.Parse ("127.0.0.1");