Test improvements

[David-Engel]

Targeting a couple of tests that fail intermittently and adding retries.

• ConnectivityTest
  • Add retries when changing the user mode of the database. I think with parallel jobs, the server may be objecting to simultaneously changing the mode of multiple databases at the same time.
  • Add a new test for connection resiliency. I wrote this for Connection Resiliency does not seem to be working #304 and Fix connection resiliency check #310 but it would still pass, even before the fix, because the async reconnect task still ran with this particular bug and always beat the subsequent query in the context of the unit test. But the test could still catch other code errors which might break connection resiliency, so it will still be good to have.
• SqlCredentialTest: This test fails intermittently with the server somehow thinking the user is not allowed to change its own password. I suspect this might also be due to other jobs trying to change passwords at the same time and adding a brief wait on failure and retrying may make it pass more consistently.