Improve test coverage for teardown with open connection

[sallner]

In #13 drop_db has been improved for cases in which the username is not identical to the db_name that has to be dropped. The execute command broke, if there is no database named like the user.

The test should roughly do the following:

• create a user
• create a db for the user with a random name
• create a connection to that database and keep it open
• call drop_db as the newly created user (without the change done in Fix force PostgreSQL drop_db. #13 this would fail because there is no database given for the psql call, so it is trying to connect to the database of the user)

Writing this test has been non-trivial as the creation and deletion of the user might temper with the ~/.pgpass which could have long-lasting effects if not properly cleared after an error in teardown. One solution might be to move default way of testing to docker as as the settings could than be made a prerequisite.