Update lmTool.test.ts to account for interface change in vscode.d.ts (#13484)

This works around a change to the vscode TextDocument interface

Author: fearthecowboy

Extension/test/scenarios/SingleRootProject/tests/lmTool.test.ts

@@ -72,13 +72,15 @@ describe('CppConfigurationLanguageModelTool Tests', () => {
             this.isClosed = isClosed;
             this.eol = eol;
             this.lineCount = lineCount;
+            this.encoding = '';
         }
         fileName: string;
         isUntitled: boolean;
         languageId: string;
         version: number;
         isDirty: boolean;
         isClosed: boolean;
+        encoding: string;
         save(): Thenable<boolean> {
             throw new Error('Method not implemented.');
         }