Write tests for New-TestResources

After various changes to fix some issues, we've accidentally regressed some scenarios for _New-TestResources.ps1_ and friends. We need some battery of basic tests to make sure that doesn't keep happening.

We can use Pester, which ships with PowerShell (no install-time dependencies then) to drive it, and should test common automation and developer scenarios, including in non-Azure SDK subs or even outside the Microsoft tenant, though I don't believe the tenant really makes a difference in this case - just access to subs for an SP. If we had to, though, we could create an MSA but maintaining it's active status could be a hassle.

/cc @benbp @pakrym @weshaggard