[REQ][Elixir] Generate ExUnit tests

[mrmstn]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I have to write my own tests based on a given OpenAPI Spec with already valid example responses.

Describe the solution you'd like

Would be pretty great if the openapi-generator would create some ExUnit Tests based on the given examples or even create some own test data based on the given type.

Additional context

Something with https://github.com/PSPDFKit-labs/bypass would be great, this would also allow us to run the tests against a non-mock server to validate the response.

Maybe https://github.com/igas/faker could also come in handy

[wing328]

We can add something like the following to the Elixir client generator:

        modelTestTemplateFiles.put("model_test.mustache", ".exs");
        apiTestTemplateFiles.put("api_test.mustache", ".exs");

Ref: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java#L70

to auto-generate test files for APIs, models.