Custom test cases should be easy to add

While the current framework supports custom test cases, it does not do so in a way that is user-friendly.

Test-AzTemplate -TestCase/-TestGroup will allow you to pass a dictionary of additional test cases and groups.

-TestCase should contain the name/ScriptBlock or Path to test
-TestGroup should contain the list of tests in each group.
Where things get annoying is that while -TestCase is additive, -TestGroup overwrites. Ergo providing your own test cases _and_ using the built in ones would be annoying.

This could be made more elegant by having something like a -TestDirectory parameter, which would read an additional directory tree's worth of tests (containing it's own /deploymentTemplate and /createUIDefinition subfolders).

Start with the Test-AzTemplate handling for -TestCase and -TestGroup. Define how an additional -TestDirectory tree containing deploymentTemplate and createUIDefinition tests should combine with built-in tests, and consider the behavior complete when custom tests and groups can be added without replacing the built-in ones.

Written by the indexing model from the issue text.