Add tests for regularization operator

Currently there're no tests for regularization operation (or at least PyCharm sees no usages).
Moreover, as it's not used in any example nor by default in optimizer.

And there're some hints that it's broken, cause it operates only on fitted subtrees, but OptGraphs by default are restored without fitted_operations that can be found only in operations cache.

Tests must be written and operator should be fixed if it doesn't work.