Make MockFileSystem's File.WriteAllText, File.ReadAllText and Directory.GetFiles support extended length paths too

Author: mostynb

src/TestableIO.System.IO.Abstractions.TestingHelpers/PathVerifier.cs

@@ -65,6 +65,12 @@ public void IsLegalAbsoluteOrRelative(string path, string paramName)
 
     private static bool IsValidUseOfVolumeSeparatorChar(string path)
     {
+        const string EXTENDED_LENGTH_PATH_PREFIX = @"\\?\";
+        if (path.StartsWith(EXTENDED_LENGTH_PATH_PREFIX))
+        {
+            path = path.Substring(EXTENDED_LENGTH_PATH_PREFIX.Length);
+        }
+
         var lastVolSepIndex = path.LastIndexOf(Path.VolumeSeparatorChar);
         return lastVolSepIndex == -1 || lastVolSepIndex == 1 && char.IsLetter(path[0]);
     }

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockDirectoryTests.cs

@@ -756,10 +756,14 @@ public async Task MockDirectory_CreateDirectory_ShouldSupportExtendedLengthPaths
 
         // Act
         var directoryInfo = fileSystem.Directory.CreateDirectory(XFS.Path(@"\\?\c:\bar"));
+        fileSystem.File.WriteAllText(@"\\?\c:\bar\grok.txt", "hello world\n");
 
         // Assert
         await That(fileSystem.Directory.Exists(XFS.Path(@"\\?\c:\bar"))).IsTrue();
         await That(directoryInfo.FullName).IsEqualTo(@"\\?\c:\bar");
+        await That(fileSystem.File.ReadAllText(@"\\?\c:\bar\grok.txt")).IsEqualTo("hello world\n");
+        await That(fileSystem.Directory.GetFiles(@"\\?\c:\bar")).HasSingle()
+            .Which.IsEqualTo(@"\\?\c:\bar\grok.txt");
     }
 
     // Issue #210