compact.js is an FSM test that runs 15 threads, each of which alternates between the following on a thread-owned collection:

1. Collection compaction
2. Collection count, confirming expected document number

While each thread has its own collection, they all share a single database. As the compact command takes a database X lock, a single thread running compact will block all others from both compact and count.

This test would better test concurrent behavior if each thread ran with its own database.