Add XCTest to Swift Testing migration guide

swift-testing/references/migration-from-xctest.md

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+# Migration from XCTest
+
+Both frameworks coexist. Migrate file-by-file. Keep XCTest for UI tests.
+
+## Conversion
+
+```swift
+// XCTest
+class UserTests: XCTestCase {
+    func testUserCreation() { }
+    func testUserValidation() { }
+}
+
+// Swift Testing
+@Suite struct UserTests {
+    @Test func creation() { }
+    @Test func validation() { }
+}
+```
+
+### Setup and Teardown
+
+```swift
+// XCTest
+class ServiceTests: XCTestCase {
+    var sut: MyService!
+    var mockRepo: MockRepository!
+
+    override func setUpWithError() throws {
+        mockRepo = MockRepository()
+        sut = MyService(repository: mockRepo)
+    }
+
+    override func tearDownWithError() throws {
+        sut = nil
+        mockRepo = nil
+    }
+}
+
+// Swift Testing
+@Suite struct ServiceTests {
+    let sut: MyService
+    let mockRepo: MockRepository
+
+    init() {
+        mockRepo = MockRepository()
+        sut = MyService(repository: mockRepo)
+    }
+}
+```
+
+### Assertions
+
+```swift
+// XCTest                              // Swift Testing
+XCTAssertTrue(condition)               #expect(condition)
+XCTAssertFalse(condition)              #expect(!condition)
+XCTAssertEqual(a, b)                   #expect(a == b)
+XCTAssertNotEqual(a, b)                #expect(a != b)
+XCTAssertNil(x)                        #expect(x == nil)
+XCTAssertNotNil(x)                     #expect(x != nil)
+XCTAssertGreaterThan(a, b)             #expect(a > b)
+XCTAssertLessThanOrEqual(a, b)         #expect(a <= b)
+XCTAssertIdentical(a, b)               #expect(a === b)
+```
+
+### Optional Unwrapping
+
+```swift
+// XCTest
+func testUnwrap() throws {
+    let value = try XCTUnwrap(optional)
+    XCTAssertEqual(value.name, "test")
+}
+
+// Swift Testing
+@Test func unwrap() throws {
+    let value = try #require(optional)
+    #expect(value.name == "test")
+}
+```
+
+### Error Testing
+
+```swift
+// XCTest
+XCTAssertThrowsError(try doSomething())
+XCTAssertThrowsError(try validate("")) { error in
+    XCTAssertEqual(error as? ValidationError, .empty)
+}
+XCTAssertNoThrow(try safeThing())
+
+// Swift Testing
+#expect(throws: (any Error).self) { try doSomething() }
+#expect { try validate("") } throws: { $0 as? ValidationError == .empty }
+#expect(throws: Never.self) { try safeThing() }
+```
+
+### Async Tests
+
+```swift
+// XCTest
+func testAsync() async throws {
+    let result = try await service.fetch()
+    XCTAssertEqual(result.count, 5)
+}
+
+// Swift Testing
+@Test func async() async throws {
+    let result = try await service.fetch()
+    #expect(result.count == 5)
+}
+```
+
+### Skipping Tests
+
+```swift
+// XCTest
+func testConditional() throws {
+    try XCTSkipIf(ProcessInfo.processInfo.environment["CI"] == nil)
+    // test code
+}
+
+func testSkipUnless() throws {
+    try XCTSkipUnless(FeatureFlags.newFeature)
+    // test code
+}
+
+// Swift Testing
+@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil))
+func conditional() { }
+
+@Test(.disabled("Not implemented yet"))
+func notReady() { }
+```
+
+### Parameterized Tests
+
+```swift
+// XCTest
+func testMultipleInputs() {
+    for input in ["a", "b", "c"] { XCTAssertFalse(input.isEmpty) }
+}
+
+// Swift Testing
+@Test(arguments: ["a", "b", "c"])
+func multipleInputs(_ input: String) { #expect(!input.isEmpty) }
+```
+
+## Checklist
+
+1. `import XCTest` → `import Testing`
+2. `class FooTests: XCTestCase` → `@Suite struct FooTests`
+3. Remove `test` prefix, add `@Test`
+4. `setUpWithError()` → `init() throws`
+5. Remove `tearDown()`
+6. `XCTAssert*` → `#expect` / `#require`
+7. `XCTUnwrap` → `try #require`
+8. `XCTSkipIf` → `.enabled(if:)` / `.disabled()`
+
+## Pitfalls
+
+```swift
+var sut: MyService!              // bad: IUO
+let sut: MyService               // good
+
+static var shared: Resource?     // bad: shared state
+let resource: Resource           // good: fresh per test
+
+XCTAssertEqual(result!.name, x)  // bad: force unwrap
+try #require(result)             // good
+```
+
+## Keep in XCTest
+
+- UI tests (XCUITest)
+- Performance tests (`measure {}`)
+- KIF / other XCTest frameworks
+