Add swift-testing compatibility docs

[MahdiBM]

See vapor/vapor#3257 and vapor/template#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.

[alemohamad]

@MahdiBM with the new merged PR, can this be closed? Thanks!

[MahdiBM]

Thank you.