Sync eng/common directory with azure-sdk-tools for PR 1210 (Azure#12545)

* Cache created service principal for iteration

Useful when testing changes over and over again without passing your own -TestApplicationId and -TestApplicationSecret.

* Restore initial AzContext for New-TestResources

* Make sure PSBoundParameters is correct

Fixes #1177

Co-authored-by: Heath Stewart <[email protected]>

Author: azure-sdk