Preprocess test data when evaluating learners

[kernc]

No description provided.

[codecov-io]

Current coverage is 90.39%

Merging #925 into master will increase coverage by +0.01% as of aeb90ed

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[lanzagar]

This uses preprocessors on test data and applies them on training data.
Learners should not get information from the test data before fitting the model as it can lead to (misleadingly) higher evaluation scores.

[kernc]

Closed in favor of #598.