integration tests should not be within unittest sections of ddbc source

[Samael]

historically ddbc has used version(unittest) within source/ddbc for tests that connect to local databases (typically in docker containers). This is not ideal, the unittest feature of D should be used for unit testing only, not integration/e2e tests. Besides which there is already the examples & ddbctest packages that are used for the same thing.

Moving forward, running dub test in the project root should correctly run passing unit tests (no database required). Contributors to the project should be directed towards running the examples prior to making a pull request.