test/integration: clean up hanging 'threads'

[alxndrsn]

Resolves need for mocha's --exit flag, which the docs say:

enables a set of potential problems; it’s indicative of tests (or fixtures,
harnesses, code under test, etc.) which don’t clean up after themselves
properly. Ultimately, "dirty" tests can (but not always) lead to false
positive or false negative results.

Flag originally introduced in:

• 472383e
• ca08b53

Problems resolved:

1. shut down db pool cleanly
2. avoid yauzl streams in cases where they hang

[alxndrsn]

I think this indicates another subtle bug in streams, probably one of:

• the cleanup code in test/util/zip.js, or
• an unfulfilled interaction that supertest is expecting when working with streams, or
• a bug in supertest's pipe/stream implementation

There's a very similar-looking issue filed at ladjs/supertest#487.