Implement PSSA2 testing

[rjmholt]

PSSA2 should be fully tested within reason.

Tests should include:

• xUnit tests of all public engine APIs
• Pester tests of all rules
• Integration tests run on full script examples with rule configurations
• Tests for picking up custom rules

As current default rules are migrated over, their Pester tests should also be migrated over to ensure those rules continue to work as intended.

Where APIs have changed, tests should test for both new and legacy APIs.

[bergmeister]

The test framework should also make it possible to unit test rules, this will highly improve future rule development