Use {withr} more comprehensively in tests to manage state

[MichaelChirico]

I couldn't remove all on.exit() usages, here's what remains:

tests/testthat/test-error.R:29:  on.exit(lintr:::reset_lang(old_lang), add = TRUE)
tests/testthat/test-get_source_expressions.R:5:    on.exit(close(con))
tests/testthat/test-object_usage_linter.R:312:  on.exit(utils::globalVariables(old_globals, package = globalenv(), add = FALSE))
tests/testthat/test-settings.R:61:  on.exit(if (is.na(old)) Sys.unsetenv(test_env) else sym_set_env(test_env, old))

Of those

• test-error.R: can't be removed unless we add a custom unexported local_lang()
• test-get_source_expressions.R: see Support UTF-8 content in local_tempfile(lines = ) withr#210
• test-object_usage_linter.R: I support we could use withr::defer(), but on.exit() seems fine here
• test-settings.R: this may be replaced by withr::local_envvar(), but I was too lazy to play around with it & ensure it works

[codecov-commenter]

Codecov Report

Merging #1646 (7b0330c) into main (51c63e0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1646   +/-   ##
=======================================
  Coverage   98.29%   98.29%           
=======================================
  Files         100      100           
  Lines        4397     4397           
=======================================
  Hits         4322     4322           
  Misses         75       75           

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[IndrajeetPatil]

Wow, that's a thorough clean-up. Thanks a lot, @MichaelChirico!

As for the remaining instances of on.exit(), we can revisit them later. 🧹