MockDirectoryInfo implement LastWriteTimeUtc

[johncmckim]

I need to use LastWriteTimeUtc on MockDirectoryInfo which is not implemented. Could this be implemented please?

[tathamoddie]

Hi! The best route to an implementation is a pull request with tests. Shout out if you need help.

[johncmckim]

No worries. I'll put something together.

[jpreese]

To be resolved by #311

[jpreese]

#311 has been merged. Closing!