am_running is true if the code is running inline tests (e.g. let%expect_test, let%test, let%test_unit) or is in an executable invoked from inline tests.
val am_running_env_var : string
val testing :
[ `Not_testing | `Testing of [ `Am_test_runner | `Am_child_of_test_runner ] ]
`Am_test_runner means the ./inline_tests_runner process, whereas `Am_child_of_test_runner means a process descended from the test runner.