Make it possible to cancel a mock local query

We're going to add tests to check that we cancel a remote query.

We'll need a `.cancel` method to be present on our mock local queries for this.

Author: elenatanasoiu

extensions/ql-vscode/src/vscode-tests/factories/local-queries/local-query-history-item.ts

@@ -47,6 +47,7 @@ export function createMockLocalQueryInfo({
   const localQuery = new LocalQueryInfo(initialQueryInfo, cancellationToken);
 
   localQuery.failureReason = failureReason;
+  localQuery.cancel = () => { /**/ };
 
   if (queryWithResults) {
     localQuery.completeThisQuery(queryWithResults);