Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

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.