test() should have an option to temporarily set env=

[MichaelChirico]

          I would like to see a similar `test(testnum, my_computed_value, my_expected_value, env=list(SOME_ENV_VAR="VALUE"))` -- I was recently working on #5807 which involves environment variables for getting default number of threads, and the tests would be much easier to understand/modify with such an enhancement to test.

Originally posted by @tdhock in #5845 (comment)