Assumption of R>4 in tests

[MichaelChirico]

Many tests in test-modules-print.R rely on an implicit assumption of R >= 4.0.0. For example:

tableone/tests/testthat/test-modules-print.R

Lines 447 to 450 in b02ef39

	expect_equal(ModuleConvertNormal(eg_mat,
	digits = 0),
	data.frame(col1 = eg_mean,
	col2 = " (2140)"))

Uses the default value of stringsAsFactors whereas the source code:

tableone/R/modules-print.R

Lines 257 to 261 in c3cf03c

	data.frame(col1 = unname(rowMat[,"mean"]),
	col2 = paste0(" (",
	ModuleFormatNumericVector(rowMat[,"sd"], digits, formatOptions),
	")"),
	stringsAsFactors = FALSE)

Explicitly sets the value.

Fixes include:

1. Bumping the version dependency to R>=4.0.0
2. Explicitly setting stringsAsFactors=FALSE in the tests