test: Add smoke-tests

[sdobbert]

This adds some smoke test to test if the docker setup is running correctly.

The testcases include:

• All services are up and running
• Databse is accessible
• Keycloak is instantiated
• Realm is instantiated with expected clients
• Realm has expected user(s)
• Login with default GeoServer credentials fails
• GeoServer has expected workspaces
• GeoServer has expected layers
• GeoServer has expected geofence rules
• GeoServer has expected version
• Client plugins index.js is accessible and contains expected plugins
• admin-client-config.js is accessible and contains expected content

The tests can be run with pytest tests/.
Requirements can be installed with pip install -r requirements.txt.
Expected values can be set in the config.yaml.

[dnlkoch]

That is awesome @sdobbert!