Make the `flutter test` tests resilient to startup lock (#8941)

packages/flutter_tools/test/test_test.dart

@@ -73,6 +73,8 @@ Future<Null> _testFile(String testName, String workingDirectory, String testDire
 
   expect(exec.exitCode, isNonZero);
   final List<String> output = exec.stdout.split('\n');
+  if (output.first == 'Waiting for another flutter command to release the startup lock...')
+    output.removeAt(0);
   output.add('<<stderr>>');
   output.addAll(exec.stderr.split('\n'));
   final List<String> expectations = fs.file(fullTestExpectation).readAsLinesSync();