.Net NEVM: Specification test suite for providers

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The current integration tests for NEVM providers are implemented separately for each provider, with no support from the abstraction. To promote better universal coverage and reduce the per-provider work needed, we could expoes a "specification test suite" with the abstraction, which providers implement, and which execute a set of standardized tests to ensure the provider works correctly. A good model here would be the Entity Framework Core specification test suite, which exercises an EF provider in a myriad of ways.

/cc @westey-m