Lrnr_lightgbm broken on Windows

[nhejazi]

On Windows, it seems that Lrnr_lightgbm not only fails relevant unit tests but causes an underlying issue that crashes the active R session. It was verified by @jeremyrcoyle that just running the simple example code at https://github.com/microsoft/LightGBM/blob/master/R-package/demo/basic_walkthrough.R#L1-L27 crashes R on Windows. This seems to be related to microsoft/LightGBM#4155 but is not really an issue with sl3. For now, we'll disable unit testing on Windows and re-enable once LightGBM has been appropriately repaired.