Add OIDC issuer validation and new testing style

[Avery-Dunn]

This PR adds issuer validation for OIDC scenarios: 3268767

• When using the oidcAuthority() API, a call is made to the OIDC discovery endpoint based on the authority set for the application
• Part of the response is an expected issuer, and if this issuer differs from what we expect based on the authority we will throw an exception.

In addition, to test the issuer validation this PR builds on the work done in #968 to add a new testing style which was first proposed in MSAL Python: AzureAD/microsoft-authentication-library-for-python#825

• Adds a new RunnerTest class containing barebones test outlines for a few scenarios, including issuer validation
• Adds a new RunnerHelper class containing helper methods for RunnerTest: retrieve test cases from a central server, instantiate objects/execute methods/validate results all based on the test config
• Adds several new classes in the testcase folder which represent and parses the content of test cases from the central server

[bgavrilMS]

This upcasting is a code smell. If you use inheritance, have you considered adding isIssuerValid to the base class Authority, where it simply returns true and have custom logic in OidcAuthority ?

[Avery-Dunn]

In the latest commit I rearranged the methods: the call to the validation method now happens in the setAuthorityProperties() method, so there are now no changes to this class and all the logic is contained within OidcAuthority

[bgavrilMS]

The test infra is not ready. Maybe have a regular test instead?

[Avery-Dunn]

The test infra is not ready. Maybe have a regular test instead?

In the latest commit I removed the POC test infrastructure and added a new OidcAuthorityTest class to handle the tests in a more traditional way.

I had mainly added it to this PR to more easily get reviews/feedback on the new style, and I'll make a follow-up PR for those changes.