[xcode11.3] [monotouch-test] Make NetworkReachabilityTest.CtorIPAddressPair a bit more permissive. Fixes xamarin/maccore#2047.

[monojenkins]

Try to fix https://github.com/xamarin/maccore/issues/2047 by making the test
accept NetworkReachabilityFlags.Reachable for the loopback address on device.

Fixes https://github.com/xamarin/maccore/issues/2047.

Backport of #7516.

/cc @rolfbjarne

[monojenkins]

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[chamons]

Approved as test only fix.

[spouliot]

Test still fails :(

https://jenkins.internalx.com/blue/organizations/jenkins/macios/detail/xcode11.3/30/pipeline

[2019-12-17T03:32:13.624Z] 1) CtorIPAddressPair (MonoTouchFixtures.SystemConfiguration.NetworkReachabilityTest.CtorIPAddressPair)
[2019-12-17T03:32:13.624Z]      #3 Reachable
[2019-12-17T03:32:13.624Z]   Expected: 0
[2019-12-17T03:32:13.624Z]   But was:  Reachable, IsLocalAddress, IsDirect
[2019-12-17T03:32:13.624Z] 
[2019-12-17T03:32:13.624Z]   at MonoTouchFixtures.SystemConfiguration.NetworkReachabilityTest.CtorIPAddressPair () [0x000fd] in /Users/builder/jenkins/workspace/xamarin-macios/xamarin-macios/tests/monotouch-test/SystemConfiguration/NetworkReachabilityTest.cs:143 
[2019-12-17T03:32:13.624Z]   at (wrapper managed-to-native) System.Reflection.RuntimeMethodInfo.InternalInvoke(System.Reflection.RuntimeMethodInfo,object,object[],System.Exception&)
[2019-12-17T03:32:13.624Z]   at System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0006a] in /Users/builder/jenkins/workspace/xamarin-macios/xamarin-macios/external/mono/mcs/class/corlib/System.Reflection/RuntimeMethodInfo.cs:395