Change tests to not modify Core_Root (#23470)

Change tests to not modify Core_Root (#23470)

Redesign tests that needed to modify Core_Root to no longer do so.

- Loader.FromNativePaths: Use `CORE_LIBRARIES` instead of `COREROOT`
- Loader.AssemblyDependencyResolver: Split out the invalid hosting test that needs hostpolicy to not be preloaded. Have `CoreRun` and `CoreShim` preload hostpolicy when a path to a hostpolicy is specified in the `MOCK_HOSTPOLICY` environment variable.
- Add a `CLRTest.MockHosting.targets` file that is imported when a test requires hostpolicy. It adds the reference to the mock hostpolicy and ensures that the `MOCK_HOSTPOLICY` environment variable is correctly set in the test scripts.

Fixes #23429.