fix: envoy shutdown flaky test

[guydc]

What this PR does / why we need it:

This PR delays readiness of envoy to allow XDS sync as discussed in #2810. This is done specifically for the upgrade test suite gateway class.

Test flakiness seems to be related to 404 responses in early stages of the load test, which likely indicates that new proxy instances are ready before being programmed.

--- FAIL: TestEGUpgrade (230.50s)
    --- PASS: TestEGUpgrade/EGUpgrade (152.90s)
        --- PASS: TestEGUpgrade/EGUpgrade/Upgrade_from_an_older_eg_release_should_succeed (152.89s)
    --- FAIL: TestEGUpgrade/EnvoyShutdown (68.06s)
        --- FAIL: TestEGUpgrade/EnvoyShutdown/All_requests_must_succeed (68.04s)
        utils.go:176: failed to create load: error 404 for http://172.18.255.208/envoy-shutdown (82 bytes)
        utils.go:185: Load completed after 0s with 2 requests, 0 success, 2 failures and 2 errors

Which issue(s) this PR fixes:
Fixes #3262

[arkodg]

LGTM thanks !

[arkodg]

the 5s delay may be helpful for this e2e, but will need to be tweaked for user environments where the xDS config is large and many envoys are connecting to the control plane at once
A more robust solution would be to use the listener level health check recently added https://gateway.envoyproxy.io/latest/api/extension_types/#healthchecksettings as a active health check in the external loadbalancer. I dont think this can be added to metal lb, but will be useful for cloud environments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.30%. Comparing base (c752b88) to head (5ea7a67).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3646      +/-   ##
==========================================
+ Coverage   68.27%   68.30%   +0.03%     
==========================================
  Files         170      170              
  Lines       20760    20760              
==========================================
+ Hits        14174    14181       +7     
+ Misses       5566     5560       -6     
+ Partials     1020     1019       -1     

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