Switch test/example dataset

[martinju]

CRAN requires all packages under Suggests to be used conditionally in all code, ref #246
Since we use the MASS package for lots of examples and tests, we need to add the cumbersome if(requireNamespace("MASS") outside all this code. By switching to using a dataset in the "datasets" package which is part of core R we can probably avoid this, as it does not seem necessary to add this package under suggests. (Possibly by mistake) we currently don't have it under Suggests, while we still use it in a test without the requireNamespace stuff see

shapr/tests/testthat/test-models.R

Lines 72 to 74 in f7d742c

	# Data -----------
	data("iris", package = "datasets")
	x_var <- c("Sepal.Length", "Sepal.Width", "Petal.Length", "Petal.Width")

Should make sure we don't need the datasets package under depends before doing this switch

[martinju]

At least anova.lm from the stats package use LifeCycleSavings from the datasets package in an example without any requireNameSpace stuff, see: https://stat.ethz.ch/R-manual/R-devel/library/stats/html/anova.lm.html
It is neither mentioned in stats DESCRIPTION https://stat.ethz.ch/R-manual/R-devel/library/stats/DESCRIPTION

Thus, I think we can safely do the same.