Ensure & is escaped in XML output. (#555)

Ensure `&` is escaped in XML output as `&`. Thanks for the reminder,
@jakepetroules.

### 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: grynspan

Sources/Testing/Events/Recorder/Event.JUnitXMLRecorder.swift

@@ -216,6 +216,8 @@ extension Event.JUnitXMLRecorder {
       "<"
     case ">":
       ">"
+    case "&":
+      "&"
     case _ where !character.isASCII:
       character.unicodeScalars.lazy
         .map(\.value)