[R] Remove stringi dependency (#6109)

* [R] Fix empty empty tests and a test warnings

* [R] Remove stringi dependency (fix #5905)

* Fix R lint check

* [R] Fix automatic conversion to factor in R < 4.0.0 in xgb.model.dt.tree

* Add `R` Makefile variable

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

Makefile

@@ -138,12 +138,14 @@ Rpack: clean_all
 	rm xgboost/remove_warning_suppression_pragma.sh
 	rm -rfv xgboost/tests/helper_scripts/
 
+R ?= R
+
 Rbuild: Rpack
-	R CMD build --no-build-vignettes xgboost
+	$(R) CMD build --no-build-vignettes xgboost
 	rm -rf xgboost
 
 Rcheck: Rbuild
-	R CMD check --as-cran xgboost*.tar.gz
+	$(R) CMD check --as-cran xgboost*.tar.gz
 
 -include build/*.d
 -include build/*/*.d