feat: Implemented MockDirectory.CreateTempSubDirectory (#964)

Implemented MockDirectory.CreateTempSubdirectory(). This implementation generates a random 6 character string with an optional user specified prefix and creates the directory in the temporary directory returned by `Path.GetTempPath()`.

The 6 random characters seems to be the algorithm used on my platform (Mac) by the System.IO implementation.

Notes on this PR
1. There's a potential temporary directory namespace of $62^6$ or about 56.8 billion. As we get closer to this limit, the likelihood of generating a duplicate increases as well as the likelihood of slowdowns of the random generation. At the extreme case, if all possible temporary directories are created, creating a random directory name will effectively become an infinite loop. I assume we don't need to handle conditions close to the limit.
2. `MockDirectory` now contains a `Random` instance to generate random temp directory names. `Random.Shared` can't be used as it was introduced in .NET 6 it it appears we are supporting earlier .NET versions.
3. `MockDirectory` is marked as `[Serializable]`, however the `Random` instance cannot be serialized. I have marked the `Random` instance as `[NonSerialized]` and created a new `Random` instance on deserializing. The state of the random number generator will be different when deserialized. I don't think this is an issue as there was no way to specify the seed of the `Random` instance when creating the `MockDirectory` instance, so there's no way to generate the same sequence of random directories again (besides a very slim chance).
4. I have had to add `<EnableUnsafeBinaryFormatterSerialization>` on the test project and `#pragma warning disable SYSLIB0011` on the deserialize method to allow testing that deserialization of a `MockDirectory` also instantiates a new random number generator. Deserialization is heavily deprecated and this is only on test classes, so I am assuming this is OK.

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

@@ -110,7 +110,17 @@ public override IFileSystemInfo CreateSymbolicLink(string path, string pathToTar
         /// <inheritdoc />
         public override IDirectoryInfo CreateTempSubdirectory(string prefix = null)
         {
-            throw CommonExceptions.NotImplemented();
+            prefix ??= "";
+            string potentialTempDirectory;
+
+            // Perform directory name generation in a loop, just in case the randomly generated name already exists.
+            do
+            {
+                var randomDir = $"{prefix}{Path.GetRandomFileName()}";
+                potentialTempDirectory = Path.Combine(Path.GetTempPath(), randomDir);
+            } while (Exists(potentialTempDirectory));
+
+            return CreateDirectoryInternal(potentialTempDirectory);
         }
 #endif
 

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

@@ -813,6 +813,37 @@ public void MockDirectory_CreateDirectory_ShouldSucceedIfTryingToCreateUNCPathSh
             Assert.IsTrue(fileSystem.Directory.Exists(@"\\server\share\"));
         }
 
+#if FEATURE_CREATE_TEMP_SUBDIRECTORY
+        [Test]
+        public void MockDirectory_CreateTempSubdirectory_ShouldCreateSubdirectoryInTempDirectory()
+        {
+            // Arrange
+            var fileSystem = new MockFileSystem();
+
+            // Act
+            var result = fileSystem.Directory.CreateTempSubdirectory();
+
+            // Assert
+            Assert.IsTrue(fileSystem.Directory.Exists(result.FullName));
+            Assert.IsTrue(result.FullName.Contains(Path.GetTempPath()));
+        }
+
+        [Test]
+        public void MockDirectory_CreateTempSubdirectoryWithPrefix_ShouldCreateDirectoryWithGivenPrefixInTempDirectory()
+        {
+            // Arrange
+            var fileSystem = new MockFileSystem();
+
+            // Act
+            var result = fileSystem.Directory.CreateTempSubdirectory("foo-");
+
+            // Assert
+            Assert.IsTrue(fileSystem.Directory.Exists(result.FullName));
+            Assert.IsTrue(Path.GetFileName(result.FullName).StartsWith("foo-"));
+            Assert.IsTrue(result.FullName.Contains(Path.GetTempPath()));
+        }
+#endif
+
         [Test]
         public void MockDirectory_Delete_ShouldDeleteDirectory()
         {