Introduce connection tests

It would really good to have two steps in the `getting started` guide:

* a `hello world` part (checking the DbFit setup and connection to your database)
* a `writing your first test` part

This PR is introducing the `hello world` part.

Tasks:
* [x] connection test for Oracle
* [x] connection test for MySql
* [x] connection test for Postgres
* [ ] connection test for DB2
* [ ] connection test for SQL Server
* [ ] add connection tests to acceptance test suite
* [ ] integrate the connection tests in the 'getting started' guide
* [ ] annotate links with relevant docs sections