Enable IsDummySqlAuthenticationProviderSetByDefault in test suite

[cheenamalhotra]

Ref PR: #3620

Test: AADAuthenticationTests.IsDummySqlAuthenticationProviderSetByDefault
[.NET Framework]

The test case is currently disabled due to limited support to load app.config in test lab.

This issue needs deep investigations into how to load app.config file with our current build/test scripts and make necessary changes to be able to test the same.

[cheenamalhotra]

Resolved in the same PR.