All tests should now have annotation for *why* they're ignored on emscripten. A few tests no longer need such an annotation as well! Closes #41299
Most of these rely on spawning processes, which is not possible in Emscripten.
This commit is an implementation of [RFC #1011][rfc] which adds an `exit` function to the standard library for immediately terminating the current process with a specified exit code. [rfc]: https://github.com/rust-lang/rfcs/pull/1011
