Replace our custom test harness with Microsoft.CodeAnalysis testing harness

[MattKotsenas]

Move from our own custom analyzer harness to the one provided by the Roslyn team (Microsoft.CodeAnalysis.Testing). This harness is a lot more powerful in that it supports automatic verification of:

• Suppressing diagnostics
• Verifying generate code analysis
• Verifying the test code is doesn't have unexpected errors (i.e. test code errors)
• Testing against multiple TFMs
• Testing with multiple versions of PackageReferences
• Baselining diagnostics via markup in the test code

Unfortunately, the harness is also quite a bit slower. On my machine test times regress from ~2 seconds to ~40 seconds. I've filed #64 to track.

In order for our test code to actually compile we need to move our Microsoft.CodeAnalysis.CSharp.Workspaces from 2.6.1 (.NET Core 2.0 era) to 3.4.1 (.NET 6 era). This does impact where our analyzer can run, so I picked the oldest supported version. We could avoid this upgrade if we either:

1. Switched away from file-scoped usings
2. Figure out Refactor tests to cover both global and namespace uses #56