Implements exit codes into the engine so tests can return their statuses.
Ideally we don't do this, and we use FIXUP logic to 'begin' and 'end' the engine execution for tests specifically.
Since realistically we're initialising the engine here we don't want to do that, since String should not require an engine startup to test a single header.
This lowers the complexity of running the unit tests and even for
physics should be possible to implement such a fix.
The base branch is hardcoded as an env variable as I couldn't find a simple
way to just get either `3.2` or `master`. But it's easy to change when we
branch off from `master` to a new stable branch, which doesn't happen often.
(There's `{{github.base_ref}}` but it's probably more verbose like
`ref/heads/master`, and only valid for PRs.)
