Add missing lock around files variable

The files dictionary could be modified and resized by another thread. Without the lock, the read is not synchronized and can cause a KeyNotFoundException.

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

@@ -156,7 +156,12 @@ private string GetPathWithCorrectDirectoryCapitalization(string fullPath)
 
             if (DirectoryExistsWithoutFixingPath(leftHalf))
             {
-                string baseDirectory = files[leftHalf].Path;
+                string baseDirectory;
+                lock (files)
+                {
+                    baseDirectory = files[leftHalf].Path;
+                }
+
                 return baseDirectory + Path.DirectorySeparatorChar + rightHalf;
             }
         }
@@ -581,4 +586,4 @@ private class FileSystemEntry
         public string Path { get; set; }
         public MockFileData Data { get; set; }
     }
-}
+}