Increase test HttpClient retries & optionally wait for resources to transition to target state

[DamianEdwards]

Would appreciate your eyes @davidfowl @radical @ReubenBond

The DatabaseMigrations sample test was flakey as the migrations project is just a console app so has no healthcheck endpoint that can be monitored before trying to call the actual app endpoint, so it would sometimes fail because the retries for the app endpoint would get exhausted before the migration had finished. Now, the test can wait for a specific resource to enter a specific state before continuing, which I use to wait for the migration app to exit before hitting the actual app endpoints.

[ReubenBond]

We should consider integrating this functionality back into the core product