Update tests

24. Update the tests/testthat/test-str_split_one.R' file.

test_that("str_split_one() splits a string", {
  expect_equal(str_split_one("a,b,c", ","), c("a", "b", "c"))
})

test_that("str_split_one() errors if input length > 1", {
  expect_error(str_split_one(c("a,b","c,d"), ","))
})

test_that("str_split_one() exposes features of stringr::str_split()", {
  expect_equal(str_split_one("a,b,c", ",", n = 2), c("a", "b,c"))
  expect_equal(str_split_one("a.b", stringr::fixed(".")), c("a", "b"))
})