Group @testable imports at the bottom, per public/rules-swift's swiftformat. A
behavior-preserving import-ordering reformat across 252 files (mostly tests).
--trailingclosures was deliberately not adopted (cosmetic, and swiftformat flags
it as caution-worthy).
A <rule abstract="true" id="x"> is a template that does not fire on its
own; a concrete rule pulls in its let bindings and assertions with
<extends rule="x"/>, prepended so the concrete rule extends rather than
replaces them.
Part of #80
