Rewrite tests with `withr`

[chainsawriot]

These things are dangerous, now I know better.

rtoot/tests/testthat/test-auth_verify.R

Lines 1 to 23 in b3980cd

	original_envvar <- Sys.getenv("RTOOT_DEFAULT_TOKEN")
	Sys.setenv(RTOOT_DEFAULT_TOKEN = "abc;user;emacs.ch")
	test_that("verify_envvar (Good case)", {
	## The cassette was created with a valid envvar
	vcr::use_cassette("envvar", {
	expect_error(capture_message(verify_envvar()), NA)
	})
	})
	test_that("verify_envvar (Good case), silent", {
	## The cassette was created with a valid envvar
	vcr::use_cassette("envvar_silent", {
	expect_silent(verify_envvar(verbose = FALSE))
	})
	})
	test_that("verify_envvar (Bad case)", {
	Sys.setenv(RTOOT_DEFAULT_TOKEN = "")
	expect_error(verify_envvar())
	})
	Sys.setenv(RTOOT_DEFAULT_TOKEN = original_envvar)

perhaps withr::local_envvar().

[chainsawriot]

Probably a better idea to do this after #178 has been dealt with.