Test logging

[jackfirth]

When test cases for complex rules fail, it can be tricky to tell why. Logging helps since the log messages show up in the test output. Going one step further, it would be nice to let tests expect log messages like so:

test: "should not refactor X because of comments"
expect-log: "my-rule: suggestion discarded because it does not preserve all comments"
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... code ...
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