Self signed cert generation for tests

[ChrisLGardner]

As part of a change to xWebAdministration (PR 307) I ported some code from xCertificate written by @PlagueHO to find certificates installed on the machine as part of the same DSC config (as detailed in #261). In this code there is some tests which generate a self signed cert using a script from Technet by an MVP. No one in the PR was particularly happy with this method of generating the self signed cert particularly because the repo didn't have any control over that script and the author didn't want to put it on the Gallery.

I recommend we include that self signed cert generating script (or a version of it) into the common test helper so that it can be maintained in a controlled way and used in any resources that make use of the Find-Certificate command or have other needs to generate self signed certs for testing purposes.

As far as actually including the script, I'd like to think we can just ask the original author if we can include it and attribute him as the author and make use of it that way.