[CSHARP] test project is generated in default sourceFolder even when sourceFolder is set

[ruimaciel]

Description

When creating a C# client with swagger-codegen-cli, if the user passes a config file with sourceFolder to any path other than the default then swagger-codegen-cli still outputs the test subproject to ./src.

Swagger-codegen version

3.0.57

Swagger declaration file content or url

N/A

Command line used for generation

{
    "sourceFolder":  "FooBar"
}

java -jar swagger-codegen-cli-3.0.57.jar generate -i ${OPENAPI_SPEC} \
    -l csharp \
    -c config.json 

Steps to reproduce

How to reproduce:

1. create config.json
2. set sourceFolder to something other than the default value of src.
3. run swagger-codegen-cli with the config

Expected result:

• store the test subproject alongside the client project (i.e., comply with sourceFolder, as per this)

Actual result:

• the test subproject is outputted to ./src.

Related issues/PRs

• https://github.com/swagger-api/swagger-codegen/pull/12432

Suggest a fix/enhancement

• Add support for testFolder, as per this.

[ruimaciel]

@mandrean as per the PR checklist, could you please take a look at PR #12432 ? Thanks!