Use proper test profile when running MD from MD

Use MONODEVELOP_TEST_PROFILE instead of XDG_*. The problem is that
setting a CWD-relative path in XDG_* affects child processes such
as xbuild in unpredictable ways.

This makes it possible to debug the XA project type addin.
[Ide] Fix overbroad XML docs comment lookup

MD was finding the Mono.Android.Platform.xml manifest file when looking
for doc comments for Mono.Android.dll.