Reorganize assembly resolution; fix error loading WindowsBase.dll

[CharliePoole]

This PR fixes #1466, which is caused by loading the wrong copy of WindowsBase.dll, the copy that's part of Microsoft.NetCore.App rather than the one from Microsoft.WindowsDesktop.App.

In order to resolve it, I did a great deal of refactoring in TestAssemblyResolver. We were using four main strategies to locate assemblies in a fixed order. I've now changed the structure as follows:

1. The strategies are now formal, using the Strategy pattern.
2. I added some heuristics to decide on the order to apply the strategies to each test assembly. For example, if the test assembly directly references either a Windows Forms or a WPF assembly, we check the Microsoft.WindowsDesktop.App directory first.

I view this as a start. In the future, we may want to get self-diagnostic reporting of how often each strategy is used. We may want to subdivide the strategies further or add new ones. We'll almost certainly want to look at efficiency.

So I'd like to get comments on this PR about the usefulness of the new structure for those future purposes in addition to the normal review of the bug fix. I've asked @veleek to do the primary review as he has worked in this area but anyone on the @nunit team is welcome to review as well.