added test for Test-DbaSpn
added test for Test-DbaSpn

@niphlod - this test takes 15 seconds to run. Are there any additional
opportunities for Mocks that you can add to speed it up on a non-domain
comp like appveyor?

SirCaptainMitch/dbatools,sqlpadawan/dbatools,wsmelton/dbatools,Splaxi/dbatools,kozloski/dbatools,dsolodow/dbatools