[MSBuild] Fixed a test that failed when the GAC wasn't available

PR #1229 added a test that relies on the GAC and fails when it isn't available, like when building on a clean machine with monolite. Skip the test in that case.

This should fix the test failure on [jenkins.mono-project.com](http://jenkins.mono-project.com/job/test-mono-mainline/63/label=debian-amd64/testReport/MonoTests.Microsoft.Build.Tasks/ResolveAssemblyReferenceTest/TestSystemDll/)