Use short, unique event store names in the kernel-backed specifications

[woksin]

Fixed

• Kernel-backed integration specs use short, unique event store names, so they no longer exceed MongoDB's database-name length limit and can be rerun against the same local kernel. (Integration specs fail when rerun against the same kernel #165)

[woksin]

CI note: the first integration run failed in when_replaying_reactors_with_once_only_handlers.integration.spec.ts ("Timed out waiting for the kernel" on the first live-delivery wait), a spec this PR does not touch. Rerunning the failed job passed (all 7 files, 53 tests). Locally, the suite passed twice in a row against one kernel container. The once-only handlers spec looks intermittently slow on the shared CI kernel; not addressed here.