Summary:
This helps forcibly end mocha tests when they hang, and print out
something that may help debug the situation.
Also add the generated static/bundle.css file to core/.gitignore.
Also, avoid using npm-packages-offline-cache when building core, by avoiding use of .yarnrc which turns it on.
Test Plan: Tested manually
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2788
