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).
Grow the Tier-2 conformance corpus with 4 Canonical XML mode cases: exclusive
drops an unused namespace and renders one at its point of use; Canonical XML
2.0 drops whitespace-only text nodes and trims surrounding whitespace. Its own
suite (ConformanceCanonicalModesTests).
Part of #105 (Tier 2)
