We currently run hundreds of tests on the platform-agnostic components of the macOS accessibility implementation, but we don't yet test that we can get the C++ node wrapper class (AXPlatformNodeMac) that corresponds to a given AXNode or that we can get the native Objective-C macOS accessibility object (AXPlatformNodeCocoa) from an AXPlatformNode.

Add an initial test that supports adding further platform-specific tests of these wrappers.