MockingFileSystem_1

 public class ProductionClass
    {
        public async Task CheckDataAndStoreResultsAsync(string inputFilePath, string outputFilePath)
        {
            using (var inputFileStream = File.OpenText(inputFilePath))
            {
                using (var outputFileStream = File.AppendText(outputFilePath))
                {
                    string inputLine;
                    int lineCounter = 1;
                    while ((inputLine = await inputFileStream.ReadLineAsync()) != null)
                    {
                        if (inputLine.Contains("OK"))
                        {
                            await outputFileStream.WriteLineAsync(lineCounter.ToString());
                        }

                        lineCounter++;
                    }
                }
            }
        }
    }

    [TestFixture]
    public class Tests
    {
        [Test]
        public void Examined_Data_Written_Correctly_To_File()
        {
            var inputFilePath = "inputFile.txt";
            var outputFilePath = "outputFile.txt";
            var inputFileContents = new[] {"OK", "WRONG", "OK", "WRONG"};
            File.WriteAllLines(inputFilePath, inputFileContents);
            File.Delete(outputFilePath);

            var sut = new ProductionClass(new FileSystemInteractor());
            var resultAsync = sut.CheckDataAndStoreResultsAsync(inputFilePath, outputFilePath);
            resultAsync.Wait();

            var outputLines = File.ReadAllLines(outputFilePath);
            CollectionAssert.AreEquivalent(new []{"1", "3"}, outputLines, $"Actual:{string.Join(",", outputLines)}");
        }
    }