JS: Spurious failures in CI

The JS backend experiences occasional failures in CI, which is holding up marge batches.

This is likely due to a combination of real bugs and poorly adjusted timeouts for the tests.

I'm going to mark the tests as js_fragile with a reference to this ticket number. In the future we can track the underlying cause here, and if necessary open new tickets for them.

Hopefully in a few months we can check back on this ticket, review the CI logs and remove the js_fragile designation if tests aren't failing anymore.