Add swift-testing compatibility docs

See https://github.com/vapor/vapor/pull/3257 and https://github.com/vapor/template/pull/147. Mention `.testable()` -> `.testing()`. Mention `XCTVapor` -> `VaporTesting`. Mention you should explicitly `import Testing`. Mention you MUST use the corresponding module or there is a good chance your Vapor test failures won't get reported.