Convert unit tests to Constraint model

[dodexahedron]

As part of migrating to NUnit 4, the biggest work item is changing all tests to use the constraint model, instead of classic assertions.

This issue is a high-level tracking item for that work.

Resulting tests should be as or more thorough than all existing tests.

Additional issues will be created for other changes to tests beyond conversions.

[dodexahedron]

TerminalGuiDesigner PR 259 opened for remote tracking of work on this.

[dodexahedron]

Note that tests will be failing for now, until some changes to the Key class being discussed at Terminal.Gui are completed and can be used here. I'm going to keep on working on other stuff anyway and just ignore the failures for now.

Looks like @tig already put up a PR to get that fixed up, so shouldn't be long.

[dodexahedron]

Since the key class has a nice and compact string representation, and to make KeyMap more of an actual configuration class, I made all of the Key properties strings and updated comparisons wherever used.

Simplified things a lot and fixed the tests.

Also, if/when implicit string conversion for Key is implemented, the ToString calls will also go away.