Convert the cross-platform swift test suites from XCTest to Swift Testing

[jkmassel]

Converts the last XCTest suites in the repo-root swift test run — the macOS cross-platform gate added in #25344 — to Swift Testing. That gate now runs 233 Swift Testing cases across 53 suites and zero XCTest cases.

What changed

38 test files across the four targets that still had XCTest, plus one deletion:

• GutenbergProcessorsTests (5), JetpackStatsWidgetsCoreTests (3), WordPressFluxTests (1), WordPressSharedTests (29). WordPressCoreTests was already Swift Testing.
• Delete WordPressFluxTests/XCTestManifests.swift — the Linux allTests() manifest is dead under Swift Testing, and was already #if !canImport(ObjectiveC) (empty on Apple platforms).

Most of it is mechanical: XCTestCase subclass → struct suite, func testX → @Test func, XCTAssert* → #expect / try #require / Issue.record, setUp → init. Four suites needed more than a rote swap.

WordPressFluxTests runs on @MainActor now

Flux's Dispatcher traps off the main thread (assertMainThread(), Dispatcher.swift:41). XCTest ran these on the main thread so they passed; Swift Testing runs on the cooperative pool, so the plain conversion aborted the entire swift test process with Assertion failed: Dispatcher should only be called from the main thread — a trap, not a single recorded failure. Pinning the suite to @MainActor restores main-thread execution.

DebouncerTests pumps the run loop instead of awaiting

Debouncer schedules a Timer on the current run loop. XCTest's wait(for:timeout:) spun that run loop; Swift Testing's await/confirmation don't, so a naive async port never fires the timer. These stay synchronous and spin RunLoop.current.run(mode:before:) until the callback lands or the deadline passes, wrapped in withExtendedLifetime — Debouncer.deinit fires the pending callback on dealloc, so releasing it early corrupts the timing. testDebouncerRunsImmediatelyIfReleased relies on that same deinit behavior and stays a pure synchronous check.

Smaller notes

• WidgetDataCacheReaderTests' assert(…line:) helper becomes sourceLocation: SourceLocation = #_sourceLocation, forwarded to #expect(…, sourceLocation:) / Issue.record(…, sourceLocation:) so failures still point at the call site.
• LazyTests stays a final class — its @Lazy property wrapper mutates and its tearDown (→ deinit) resets static state, both wanting reference semantics.
• XCTAssertEqual(x.count, 0) and XCTAssertEqual(y, "") became #expect(x.isEmpty) / #expect(y.isEmpty): the == 0 / == "" forms trip SwiftLint's empty_count / empty_string, which the XCTAssertEqual argument form did not.
• Dropped a disabled xtestPerformance case — XCTest measure has no Swift Testing equivalent.

The relocated suites (#25815, #25825)

After this branch was first written, #25815 (the KeystoneTests move) and #25825 (three Foundation extensions) relocated 15 more XCTest suites into WordPressSharedTests. The branch is rebased on top of them and converts those too, so the gate stays entirely Swift Testing instead of re-gaining XCTest the moment it merges. Two needed care: NotificationCenterObserveOnceTests is pinned to @Suite(.serialized) — its two tests share NotificationCenter.default and one observes without a filter, so under Swift Testing's default parallelism it would catch the other's posts; and QueueTests stays a final class because Queue is a value type it mutates across each test.

The suite runs faster, too

Runner-reported execution time drops from ~1.6s to ~0.5s for the same 233 tests (one warm run: 1.56s → 0.52s). Swift Testing parallelizes by default, whereas swift test runs XCTest serially within the bundle. The win is entirely DebouncerTests: its four timer-based cases sleep ~1.4s in total and ran serially under XCTest (Executed 4 tests … in 1.414 seconds), which by itself was most of the before-run. Swift Testing overlaps them with each other and the other 229 tests, so the run collapses to about its single slowest test. Not the goal of the change, just a free improvement to the gate.

Not in this PR

• RichContentFormatterTests.swift and WPUserAgentTests.swift are exclude:d from the macOS WordPressSharedTests target (they don't build on macOS), so they're outside the swift test set and stay XCTest.
• Pre-existing swift-format warnings in the Gutenberg fixtures (column-0 multiline HTML strings) are left as-is; reformatting them is unrelated churn. This PR introduces no new swift-format warnings.

Test plan

• swift test from the repo root: 233 Swift Testing cases in 53 suites pass; the XCTest runner reports Executed 0 tests.
• rake lint[<changed files>]: 0 violations.
• swift-format lint on the changed files: no new warnings — the three fully-rewritten suites (DebouncerTests, StringRankedSearchTests, URLIncrementalFilenameTests) went from 5/4/6 warnings to 0.
• CI (Buildkite build #33419) compiles and runs these files on the simulator via Modules/Package.swift (WordPressUnitTests.xctestplan): the WordPress + Jetpack Build for Testing and Unit Tests jobs are green — the conversions run on iOS, not just macOS.

[dangermattic]

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	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	33419
	Version	PR #25828
	Bundle ID	com.jetpack.alpha
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