[csharp] Use the provided 'sourceFolder' property to generate the test folder

[Starkie]

Hi all!

This issue is regarding the csharp-netcore generator. There is a bit of an inconsistency in how the test folders are generated when the sourceFolder property is provided. For example:

• Set the option sourceFolder to anything other than src.
  • In this case, I left it empty: --additional-properties=sourceFolder=""
• Generate the client
• See that the client was generated in the provided sourceFolder... but the tests where generated in src:
  

In the AbstractCSharpCodegen.java, there is a TODO a few years old saying that a separate option for testFolder could be added:
https://github.com/OpenAPITools/openapi-generator/blob/8551b0af492286a431c95ce3a37d7edf1ef4870a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java#L73-L75

Until that is implemented, I created a little workaround, assigning the provided sourceFolder parameter to the testFolder.

With this change, the projects are generated as expected:

Related issue: OpenAPITools#1390

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