PInvokes into hostpolicy.dll (which should live next to the runtime and thus always be reachable).
If the PInvoke fails (missing hostpolicy.dll) we will fail for now.

Adds tests for the API into CoreCLR repo. The main reason is that with corerun
we can easily mock the hostpolicy.dll since there's none to start with.
Writing the same tests in CoreFX or any other place which starts the runtime through
hostpolicy would require test-only functionality to exist in either the class itself
or in the hostpolicy.

 Add native resolution tests

Registers error writer with the hostpolicy to receive detailed errors. Uses that in the exception.

Modifications to the mock and the tests to be able to verify the functionality.