test: fix flaky RouteStatNameTest and refactor

[Arpit529Srivastava]

What type of PR is this?

fix flaky e2e tests

What this PR does / why we need it:
httproutestatnametest/prometheus and tcproutestatnametest/prometheus were flaky because they sent traffic immediately after the gateway and route were accepted, without waiting for the backendtrafficpolicy (which configures routestatname) to be accepted and propagated to envoy via xds.

this introduced a race condition where traffic could reach envoy before the custom stat name was configured, so the expected prometheus metric label was never emitted and the 1-minute poll failed with context deadline exceeded.

the fix adds a backendtrafficpolicymustbeaccepted check in both tests before sending traffic, ensuring the policy is fully reconciled and pushed to envoy first. this follows the same pattern already used by other tests in this package.

this pr also adopts the refactoring from #8350 by merging both tests into a single routestatnametest with t.run("httproute") and t.run("tcproute") subtests.
Which issue(s) this PR fixes:

Fixes #8110

Release Notes: Yes/No
No

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[Arpit529Srivastava]

@zhaohuabing Ptal, thanks :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.20%. Comparing base (cd89ca5) to head (80f9f0e).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8368      +/-   ##
==========================================
- Coverage   74.23%   74.20%   -0.04%     
==========================================
  Files         242      242              
  Lines       37335    37335              
==========================================
- Hits        27717    27703      -14     
- Misses       7685     7696      +11     
- Partials     1933     1936       +3     

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[Arpit529Srivastava]

again flaky #7484 ?
/cc : @zhaohuabing

[zirain]

@Arpit529Srivastava can you adpot #8350 into your PR?

[Arpit529Srivastava]

@Arpit529Srivastava can you adpot #8350 into your PR?

made the requested changes please take a look, thanks :)

[Arpit529Srivastava]

/cc @zirain

[zirain]

wait #8370

[zirain]

still flaky https://github.com/envoyproxy/gateway/actions/runs/22521687092/job/65272562922?pr=8368

[Arpit529Srivastava]

@zirain i don’t think this failure is related to my changes. this pr only touches stat_name.go and its testdata.

the HeaderBasedConsistentHashLoadBalancing test is known to be flaky (see #8349), so this looks like the same issue. could we try re-running the ci?

[zhaohuabing]

LGTM — thanks for helping fix the flake!

[zirain]

/retest

[Arpit529Srivastava]

@zhaohuabing @zirain ig we are good to merge?