Need an analyzer for validating test blocks have an Execute method

Right now, we only want to support test blocks with one and only one Execute method. We need an analyzer for that.

Eventually there might be a need for multiple Execute methods, or alternative methods. If that ever materializes, we'll need to find a way to update the analyzer, and maybe add some messaging suggesting that test blocks probably should have one and only one execute method to avoid confusion on what's being executed in any given scenario.

The issue names no files, tests, or entry points. Locate the representation of test blocks and the analyzer registration, then inspect how analyzer tests are organized; done means test blocks with exactly one Execute method are accepted and other counts are reported clearly.

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