[tests][linksdk][watchos] Fix HttpClientHandler tests for CFNetwork/watchOS

[spouliot]

As noted in PR 177 [1] this does not build on watchOS since CFNetwork
is not supported on that platform. Uncaught as the PR bots don't build,
nor run, the tests yet.

[1] #177

[xamarin-release-manager]

Build success

[akoeplinger]

@spouliot this doesn't fix the build error in make -C tests "build-watchos-sim-link sdk" V=1:

Errors:

/Users/alexander/dev/xamarin-macios/xamarin-macios/tests/linker-ios/link sdk/link sdk-watchos.csproj (Build) ->
/Users/alexander/dev/xamarin-macios/xamarin-macios/_ios-build/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/iOS/Xamarin.iOS.Common.targets (_ResolveWatchAppReferences target) ->
/Users/alexander/dev/xamarin-macios/xamarin-macios/tests/linker-ios/link sdk/link sdk-watchos-app.csproj (default targets) ->
/Users/alexander/dev/xamarin-macios/xamarin-macios/_ios-build/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/iOS/Xamarin.iOS.Common.targets (_ResolveAppExtensionReferences target) ->
/Users/alexander/dev/xamarin-macios/xamarin-macios/tests/linker-ios/link sdk/link sdk-watchos-extension.csproj (default targets) ->
/Library/Frameworks/Mono.framework/Versions/4.4.0/lib/mono/4.5/Microsoft.CSharp.targets (CoreCompile target) ->

    HttpClientHandlerTest.cs(46,29): error CS0246: The type or namespace name `CFNetworkHandler' could not be found. Are you missing an assembly reference?

     3 Warning(s)
     1 Error(s)

Time Elapsed 00:00:01.3508830
make: *** [build-watchos-sim-link sdk] Error 1

[spouliot]

@akoeplinger added wrong file :( should be fixed now

[akoeplinger]

It is, thanks :)