fix io_test (flutter#48097)

Author: dnfield

packages/flutter_tools/test/general.shard/base/io_test.dart

@@ -77,6 +77,7 @@ void main() {
     setExitFunctionForTests((int value) {});
 
     expect(() => exit(0), returnsNormally);
+    restoreExitFunction();
   });
 
   test('test_api defines the Declarer in a known place', () {