Auto merge of #6900 - jethrogb:nonconcurrent-tests, r=ehuss

Fix nonconcurrent tests

The cargo testsuite relies on a clean test “root” for every test (i.e. `#[test]`-annotated function). It relied on the `test` crate's behavior to spawn a new thread for each test, which isn't done when tests aren't run concurrently, breaking the test suite. In this PR, I'm using backtraces to figure out which test is being run, which is much more robust. I also cleaned up the root initialization logic so that it no longer recursive calls the `init` function.

Fixes #6746