Increase test coverage of rename

- Invoke the rename request from multiple marker locations within a test file
- Add more test cases to test compound decl name references

Author: ahoppen

Sources/SKTestSupport/TestSourceKitLSPClient.swift

@@ -336,6 +336,11 @@ public struct DocumentPositions {
     }
     return position
   }
+
+  /// Returns all position makers within these `DocumentPositions`.
+  public var allMarkers: [String] {
+    return positions.keys.sorted()
+  }
 }
 
 // MARK: - WeakMessageHelper