Update MigratingFromXCTest making it clear you must import Testing. (#158)

Update MigratingFromXCTest to make it clear one must `import Testing`

### Motivation:

Other guides mention that one has to `import Testing`, e.g. `DefiningTests`, but readers will see `MigratingFromXCTest` before they see `DefiningTests`, and "following along" while reading makes code not compile (since **must** import Testing).

### Modifications:

Made it clear that with XCTest one needs `import XCTest` and now with swift-testing one needs to `import Testing`.

### Result:

A more clear guide, that is easier to follow along.

### Checklist:

- [x] Code and documentation should follow the style of the [Style Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should be updated.

Author: Sajjon

Sources/Testing/Testing.docc/MigratingFromXCTest.md

@@ -27,6 +27,30 @@ Before the testing library can be used, it must be added as a dependency of your
 Swift package or Xcode project. For more information on how to add it, see the
 [Getting Started](doc:TemporaryGettingStarted) guide.
 
+### Importing the testing library
+
+XCTest and the testing library are available from different modules. Instead of 
+importing the XCTest module, import the Testing module:
+
+@Row {
+  @Column {
+    ```swift
+    // Before
+    import XCTest
+    ```
+  }
+  @Column {
+    ```swift
+    // After
+    import Testing
+    ```
+  }
+}
+
+A single source file can contain tests written with XCTest as well as other 
+tests written with the testing library. Import both XCTest and Testing if a 
+source file contains mixed test content.
+
 ### Converting test classes
 
 XCTest groups related sets of test methods in test classes: classes that inherit