Update tests to generate files for multiple serializer types

[josh-burton]

Use case

It would be really useful to be able to have a single test case such as discriminated_one_of.3.0, which generates and verifies files for multiple serializers within the same test case.

Proposal

Create a new SWPTestConfig more suitable to tests. This would be used to setup the test and construct a final SWPConfig.

• add a List<> jsonSerializer field to SWPTestConfig
• add all other SWPConfig options to SWPTestConfig except for outputDirectory, and schemaPath.
• update e2eTest function to generate test files in a subfolder with the serializer name