work around sequential schedule validation limit (#2655)

* remove readalltext

* remove unused method

* remove fileexists

* remove folderexists

* remove readalllines

* remove fake local file system

* show playlist name in playout build errors

* add basic sequential schedule validator tests

* work around sequential schedule validation limit

ErsatzTV.Scanner/Program.cs

@@ -1,3 +1,4 @@
+using System.IO.Abstractions;
 using Bugsnag;
 using Bugsnag.Payload;
 using Dapper;
@@ -47,6 +48,7 @@ using Microsoft.IO;
 using Serilog;
 using Serilog.Events;
 using Serilog.Formatting.Compact;
+using Testably.Abstractions;
 using Exception = System.Exception;
 using IConfiguration = Bugsnag.IConfiguration;
 
@@ -252,6 +254,8 @@ public class Program
                 services.AddSingleton<IScannerProxy, ScannerProxy>();
                 services.AddSingleton<ILanguageCodeCache, LanguageCodeCache>();
 
+                services.AddSingleton<IFileSystem, RealFileSystem>();
+
                 services.AddMediatR(config => config.RegisterServicesFromAssemblyContaining<Worker>());
                 services.AddMemoryCache();