[6.1] Add an overview of test serialization to the XCTest migration guide (#965)

- **Explanation**:
Updates the article on migrating tests from XCTest to tell developers
about test serialization.
  - **Scope**:
Documentation updates.
  - **Issues**:
#863 
  - **Original PRs**:
#960 #963 
  - **Risk**:
Purely documentation updates, no risk to the release.
  - **Testing**:
Build documentation and read Migrating a test from XCTest.
  - **Reviewers**:
@grynspan @stmontgomery

Author: iamleeg

Sources/Testing/Testing.docc/MigratingFromXCTest.md

@@ -273,7 +273,7 @@ with optional expressions to unwrap them:
 
 ### Record issues
 
-Finally, XCTest has a function, [`XCTFail()`](https://developer.apple.com/documentation/xctest/1500970-xctfail),
+XCTest has a function, [`XCTFail()`](https://developer.apple.com/documentation/xctest/1500970-xctfail),
 that causes a test to fail immediately and unconditionally. This function is
 useful when the syntax of the language prevents the use of an `XCTAssert()`
 function. To record an unconditional issue using the testing library, use the
@@ -692,6 +692,56 @@ of issues:
   }
 }
 
+### Run tests sequentially
+
+By default, the testing library runs all tests in a suite in parallel. The
+default behavior of XCTest is to run each test in a suite sequentially. If your
+tests use shared state such as global variables, you may see unexpected
+behavior including unreliable test outcomes when you run tests in parallel.
+
+Annotate your test suite with ``Trait/serialized`` to run tests within that
+suite serially:
+
+@Row {
+  @Column {
+    ```swift
+    // Before
+    class RefrigeratorTests : XCTestCase {
+      func testLightComesOn() throws {
+        try FoodTruck.shared.refrigerator.openDoor()
+        XCTAssertEqual(FoodTruck.shared.refrigerator.lightState, .on)
+      }
+      
+      func testLightGoesOut() throws {
+        try FoodTruck.shared.refrigerator.openDoor()
+        try FoodTruck.shared.refrigerator.closeDoor()
+        XCTAssertEqual(FoodTruck.shared.refrigerator.lightState, .off)
+      }
+    }
+    ```
+  }
+  @Column {
+    ```swift
+    // After
+    @Suite(.serialized)
+    class RefrigeratorTests {
+      @Test func lightComesOn() throws {
+        try FoodTruck.shared.refrigerator.openDoor()
+        #expect(FoodTruck.shared.refrigerator.lightState == .on)
+      }
+      
+      @Test func lightGoesOut() throws {
+        try FoodTruck.shared.refrigerator.openDoor()
+        try FoodTruck.shared.refrigerator.closeDoor()
+        #expect(FoodTruck.shared.refrigerator.lightState == .off)
+      }
+    }
+    ```
+  }
+}
+
+For more information, see <doc:Parallelization>.
+
 ## See Also
 
 - <doc:DefiningTests>