add basic test for logger

I want a "pluggable" logger, but we need more tests beforehand ;-)
It's a good idea to put the logfile contents retrieval into `_get_log()` :+1:

This way however we might also spare the value assignment step for `$content`, by moving the `_get_log()` call into the test. Of course, this would only make sense, if there are no plans currently to use the variable in more than one test within each block.

I'm fine with both variations though, just curious about the intent.