* Tests: fix minasync Task<T> to derive from Task

* Tests: provide MinAsyncCorlibRef

This combines the async features of minasync with mincorlib to produce
a minimum unversioned corlib with async stubs.

* Compilation: use System.Object from target corlib

When creating a script compilation without an explicit return type,
System.Object was being resolved via reflection from the host.

This resulted in an implicit dependency of a script compilation on the
host corlib, even if a different corlib was specified as a reference
for the compilation (e.g. Xamarin.iOS).

Fix this by using System.Object as defined in the corlib resovled
for the compilation.