added tests to increase R code coverage for sparse matrices

[StrikerRUS]

81% -> 88%.

Related to #269.

[mlampros]

@StrikerRUS,

lately I search for classifiers (xgboost, catboost, ligthgbm) and I saw that both you and jameslamb are very active in the lightgbm repository.

Thanks for adding the tests, last week I came across the following blog post which explains the differences between class(), inherits() and is() in R. Just a comment as I saw you've used class in the added tests.

[StrikerRUS]

@mlampros Thank you very much for your review and the link! These new tests are copy-pasted existing ones with slight modifications for another sparse type (the biggest change is adding as(matrix, "RsparseMatrix") to test the dgRMatrix class).

[StrikerRUS]

@jameslamb You may be interested in that blog post 🙂 .

[jameslamb]

@jameslamb You may be interested in that blog post 🙂 .

loved it! I have known bits and pieces of this but it's a nice, concise description in one place.