Allow fast tests to execute during standard build

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.