fix a test flake on appveyor (#16479)

* fix a test flake on appveyor

* only catch FileSystemExceptions

packages/flutter_tools/test/commands/analyze_once_test.dart

@@ -33,7 +33,12 @@ void main() {
     });
 
     tearDownAll(() {
+      try {
         tempDir?.deleteSync(recursive: true);
+      } on FileSystemException catch (e) {
+        // ignore errors deleting the temporary directory
+        print('Ignored exception during tearDown: $e');
+      }
     });
 
     // Create a project to be analyzed

packages/flutter_tools/test/commands/create_test.dart

@@ -42,7 +42,12 @@ void main() {
     });
 
     tearDown(() {
+      try {
         temp.deleteSync(recursive: true);
+      } on FileSystemException catch (e) {
+        // ignore errors deleting the temporary directory
+        print('Ignored exception during tearDown: $e');
+      }
     });
 
     // Verify that we create a project that is well-formed.

packages/flutter_tools/test/commands/upgrade_test.dart

@@ -46,7 +46,6 @@ void main() {
         temp.deleteSync(recursive: true);
       });
 
-
       testUsingContext('in project', () async {
         final String projectPath = await createProject(temp);
         expect(findProjectRoot(projectPath), projectPath);