Move slow tests to integration test phase, enable tests by default

Use TestNG's test groups feature to distinguish between fast, self contained unit tests and slow integration tests that access external services (`groups="integration"`).
Move the latter ones to the integration test phase and disable them by default.

Personally, **I'd prefer to have all tests enabled by default** and point out to the option that you can disable the integration tests with `-DskipITs` if you cannot fulfil the build requirements. If you agree, I'd like to amend this PR.