Add interface level tests for FunctionMetadataGenerator and SourceGenerator

Currently, we have separate set of tests for both Metadata generators - [FunctionMetadataGenerator](https://github.com/Azure/azure-functions-dotnet-worker/tree/main/test/FunctionMetadataGeneratorTests), [SourceGenerator](https://github.com/Azure/azure-functions-dotnet-worker/tree/main/test/Sdk.Generator.Tests/FunctionMetadataProviderGeneratorTests). We need interface level tests for those functionalities which should exist in both of them. This will also help in keeping consistency between them if we are changing behavior in one.

Compare the existing tests in test/FunctionMetadataGeneratorTests and test/Sdk.Generator.Tests/FunctionMetadataProviderGeneratorTests. Identify the functionality both generators are expected to provide, then design interface-level coverage that exercises those shared expectations. Done means the common behavior is tested consistently across both generators.

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