How do I perform a tap and drag in Xcode UI Test scripts?

You can use XCUICoordinate to tap and drag elements in UI Testing. Here is an example of how to pull to refresh via table cell coordinates.

let firstCell = app.staticTexts["Adrienne"]
let start = firstCell.coordinateWithNormalizedOffset(CGVectorMake(0, 0))
let finish = firstCell.coordinateWithNormalizedOffset(CGVectorMake(0, 6))
start.pressForDuration(0, thenDragToCoordinate: finish)

If you don't have elements to reference you should be able to create arbitrary coordinates based off of XCUIApplication.

let app = XCUIApplication()
let fromCoordinate = app.coordinateWithNormalizedOffset(CGVector(dx: 0, dy: 10))
let toCoordinate = app.coordinateWithNormalizedOffset(CGVector(dx: 0, dy: 20))
fromCoordinate.pressForDuration(0, thenDragToCoordinate: toCoordinate)

UI Testing doesn't have official documentation, but I've scraped the headers and put together an XCTest Reference if you are interested in more details.

Or as of Swift 4:

let view = window.staticTexts["FooBar"]
let start = view.coordinate(withNormalizedOffset: CGVector(dx: 10, dy: 20))
let finish = view.coordinate(withNormalizedOffset: CGVector(dx: 100, dy: 80))
start.press(forDuration: 0.01, thenDragTo: finish)