Add capability to LogLevelModifier to add a log appender for a test case

To be able to fine tune the log output of a particular test, the LogLevelModifier can be used: it allows to change the log levels and together with the LogDumper allows to dump it in case of a failure.

This depends on the logback configuration that is set for (all) the tests however. What would be nice is if you could add eg a console appender for a particular test to see its log output (eg on travis/jenkins)