Add test coverage for AxHost related classes

We have some tests, but there are significant gaps. Any additional coverage here will help keep this code in good shape for the next few decades. :)

Help here is appreciated. Live on the bleeding edge of the mid-nineties. :)

Supporting this scenario is important for Windows Forms customers with legacy dependencies. This code is how Visual Basic 6 (VB6) controls are supported, and we plan to add specific tests for that scenario as well. See #8253.

• Media Player https://github.com/dotnet/winforms/pull/8216 and https://github.com/dotnet/winforms/pull/8251
• Microsoft Web Browser https://github.com/dotnet/winforms/pull/11137
• System Monitor Control https://github.com/dotnet/winforms/pull/10675