Fix slow test startup

[paskozdilar]

References to other Issues or PRs

Fixes #5244

Have you read the Contributing Guidelines?

Yes.

Brief description of what is fixed or changed

The waitForGateway method waited 10 seconds in a loop for healthzServer to return StatusOK.
The healthzServer function returns StatusOK only if conn.GetState() is connectivity.Ready - but that never happened because connection stays idle before the first method call.
To resolve this, if we detect that the connection is idle, we call any method (in this case, we chose "/grpc.health.v1.Health/Check" as it seemed like an appropriate name) and ignore its result, just to get the connection started.

This fixes the test startup delay.

Other comments

[johanbrandhorst]

Great detective work here! Looks like there's a minor bazel diff, after that is fixed we can merge this :).

[paskozdilar]

Done.

[johanbrandhorst]

Thanks for your contribution!

[paskozdilar]

Thank you for your patience and support :)