Assorted test cleanup (#3673)

* test: Remove unused `assert.UTCDate`

* test: Replace `equalBuffers` with `assert.deepStrictEqual`

`spit` isn’t defined.

* cleanup: Replace additional helper in test with stdlib `assert.rejects`

* cleanup: Merge unused test suite `uncaughtException` handler into first one

* cleanup: Remove unused `Suite#testAsync`

* cleanup: Remove now-redundant `unhandledRejection` listener in tests

All versions of Node (≥16.x) supported by pg 8 default to throwing.