Create test cases for Elytron Principal Transformers

Create test cases for following Elytron Principal Transformers:

• constant-principal-transformer
• regex-principal-transformer
• regex-validating-principal-transformer
• chained-principal-transformer

Test case for aggregate-principal-transformer cannot developed due to WFCORE-2547. Once meaning of aggregate-principal-transformer will be revisited then appropriate test case will be added.