Testthat 3e: better test isolation

[czeildi]

Try to follow newer best practices, mostly related to https://testthat.r-lib.org/articles/test-fixtures.html

Description

The changes are across these general themes:

• each test should be a self contained unit and clean up after itself in the safest and most-cran compatible way
  • setup code is either in helper file or within test
  • running the test should not change landscape (no library or require)
  • even temp files only live for single test scope
  • test gists are deleted even with partial interactive run
• silence informational messages during test run

Note that to run tests interactively, devtools::load_all() is needed, which also loads helper files

I added one suggested dependency: withr which is used in tests. I believe that it is a relatively light and stable dependency, but adds safety to cleanups.

Related Issue

Closes #183

[czeildi]

This is a bigger PR but I hope that the individual commits have a scope that is digestable. That said, tell me if I should split this up into multiple PRs, or simply revert some of the changes.

[czeildi]

I converted to draft PR as there are check failures on Windows, will look into them.

[mikemahoney218]

Thank you so much! This is absolutely fantastic. Having it all in one PR is fine, and adding withr to Suggests is fine by me.

I might take a little while to review this PR -- it's a particularly busy period for my day job. That said, this looks fantastic based on my quick skim. Thank you again!

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