Fix test failure when gtk-sharp2 is not a GAC package

When gtk-sharp-2.0 is loaded from pkg-config an additional "Package" tag is written to the csproj (see e.g. http://www.mergely.com/f1U30MI8/).
We need to either strip this out or ignore the test in that case, otherwise the XML won't match the expectation and the test would fail.

//cc @directhex