Set up tests for my stack

My project is <one sentence>, built with <stack> and hosted on <host>.
It has no tests.

Set up the test tool people normally use with this stack:
- Installation, and the single command that runs everything.
- One real test of the thing I'd most hate to break: <describe it>.
- How to run a single test on its own.
- Whether tests need a database, and how to keep them away from the
  real one.

Tests run on my machine or in CI, never on the live host.