* Issue: When using the 'dart'-booster, the lgb.cv()- and lgb.train()-function would trow a warning regardless of the specified model. See issue microsoft#6612

* Fix: The associated functions have been fixed by adding a nested if-statement that only triggers if the dart-booster is specified with early_stopping_rounds != NULL.

* Unittests: The unit-test in test_parameters.R have been rewritten in the following way

	* The lightgbm()-test have had a expect_no_warning()-clause added if the model is correctly specified.

	* A new unittest have been created that checks wether lgb.cv() throws a warning errorneously.

This commit closes microsoft#6612. 🔧

Co-authored-by: James Lamb <jaylamb20@gmail.com>

* expect_no_warning() can't be found on R 3.6.0, so an attempt
with expect_silent() will be made.

* Added spaces after if-statements in lgb.cv() and lgb.train()
* Reduced the character length of unittests to 120 characters