Needs random tests in the full test suite.

It's good to tell people the input that resulted in the incorrect output, like this:

XCTAssertEqual(5, litres(11.8), "Incorrect result for hours: 11.8")

Or, in a random test:

let hours = Double.random(in: 0.1 ..< 12.0)
let expected = (insert math here)
XCTAssertEqual(expected, litres(hours), "Incorrect result for hours: \(hours)")