… strings

The integration test framework that waits for transitions and
(optionally) takes actions on transitions allows to match patterns.

If one uses a RegExp pattern than the framework only checks whether a
line contains the given RegExp pattern.

If one uses a String pattern it matches it exactly.

=> We add a `Barrier.contains()` and `Multiple.contains()` that allow
matching a line with if it contains the String (just like in RegExp)

=> This makes tests simpler as they don't have to know about the exact
padding of progres bar etc. Those may be irrelevant for the purpose of
the integration test and only complicate it.