fix node 18 type resolution error

After updating to @types/node 18.x, we started getting an error when
mocking FileWithHandle. For some reason, the type hints don't allow
for partial implementation anymore.

Hack around the problem by just casting the partial implementation.

src/explorer/sagas.test.ts

@@ -204,7 +204,7 @@ describe('handleExplorerImportFiles', () => {
     };
 
     const mockPythonFile = (name: string, contents: string) => {
-        return mock<FileWithHandle>({
+        return mock<FileWithHandle>(<FileWithHandle>{
             name,
             type: '',
             text: () => Promise.resolve(contents),
@@ -212,7 +212,7 @@ describe('handleExplorerImportFiles', () => {
     };
 
     const mockZipFile = (name: string, contents: ArrayBuffer) => {
-        return mock<FileWithHandle>({
+        return mock<FileWithHandle>(<FileWithHandle>{
             name,
             type: 'application/zip',
             arrayBuffer: () => Promise.resolve(contents),