Test reorganization/backfill for osquery runner and instance

[RebeccaMahany]

• Moves osquery instance tests into their own file
• Adds TestRunnerHandlesImmediateShutdownWithMultipleInstances to validate shutdown timing
• Adds a couple small osquery instance tests (TestHealthy_DoesNotPassForUnlaunchedInstance, TestQuery_ReturnsErrorForUnlaunchedInstance, Test_healthcheckWithRetries)
• It's no longer possible for the knapsack to be nil inside the osquery instance, so I removed those checks
• Refactored the healthcheck with retries into its own function for readability