Wait for Gateway to be ready before using it

[trepel]

Overview

This is a small try to further stabilze the test_preexisting_auth, see
#427

Verification Steps

Execute

flags="-vvv --standalone" make testsuite/tests/singlecluster/authorino/operator/sharding/test_preexisting_auth.py

and

flags="-vvv --standalone" make testsuite/tests/singlecluster/authorino/operator/sharding/test_sharding.py

Both should pass. We will see in nightlies whether this actually helped or not.

[azgabur]

I tried to verify with running more tests concurrently as this is probably the race condition that triggers this bug

flags=-n20 make authorino-standalone

Sadly the test_preexisting_auth still occasionally fails so the problem must be somewhere deeper. I remember from talks with @pehala that this test is fundamentally wrong so maybe we need to look at the test case itself to determine if it is something we even need to test or if it has really been a bug in Authorino this whole time.

[trepel]

@azgabur Thanks for review! I think that this change makes logical sense anyway so I will merge this one. About the test itself please add what you know/recall that might help to #427