Because tests aren't meant to be run on the CI builder, testing
dependencies aren't installed. `fork-ts-checker-webpack-plugin` still
was trying to type-check the test files though, and wasn't finding the
types for them. This changes the `fork-ts-checker-webpack-plugin` config
to ignore test files entirely.
Tested by manually in Docker with `scripts/build-frontend.sh`.
Thanks for your help @tw4l!
Fixes build regression introduced in
https://github.com/webrecorder/browsertrix-cloud/pull/1407
f272c608b8