Introduce escalus_ct module
Use Escalus-specific config file

That's a slimmed down version of test.config from ejabberd_tests.
What is ejabberd_domain used for? Is it even used? It doesn't seem so.

In case Escalus is used in standalone mode, the config file must be set.
Assume Common Test is available
Add standalone (outside Common Test) usage prerequisites
Fix escalus_ct:is_ct_available/0

Somehow, the automatic discovery of the application when using
`application:get_env/1` didn't work.
Extract app name in escalus_ct to a macro
Handle relative config file path properly