Add a standard folder name for tests

[trivikr]

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

The PR #434 adds src as a standard for source files in CodegenUtils. There is not standard for test folder yet.

Describe the solution you'd like

Add the following in CodegenUtils:

public static final String TEST_FOLDER = "test";

And reuse it wherever tests are generated, like in protocol tests.