Update `testem` to v1.18.1
Update `testem` to v1.18.1

This makes the `"mode": "ci"` in `testem.js` actually work...

glimmerjs/glimmer-vm,tildeio/glimmer,glimmerjs/glimmer-vm,tildeio/glimmer,glimmerjs/glimmer-vm,tildeio/glimmer