Support setting filter= by an environment variable #2280
Description
It would be nice if there was a standard environment variable that could be set and interpeted by test_dir(filter=) to facilitate filtering tests run at more scale, e.g. across many packages to run with TESTTHAT_TEST_FILTER=plot to run all tests in files like 'test.*plot.*' across a repo of packages.
Context: I'm trying to implement --test_filter for R in bazel: