simpletest not running init hook in test module

I'm testing the new Backdrop release against some changes in our general system setup, and right on the very first test, it's behaving very strangely. The Actions test enables a test module, actions_loop_test, that depends on code running in its hook_init implementation. But that hook never gets called. I've tried the same test in a new generic installation and it works fine. So it must be something about how our site is configured. But I can't even imagine what could have this kind of effect. It looks like the test page it's attempting to run is running in the standard site environment? In the test code, system, dblog, and actions_loop_test implement the hook, but from a debug call inside dblog_init(), when the test page runs, all the usual modules are there.

Are there config values or things in the settings.php file that might be messing with the tests, outside of what simpletest controls?