[fuzz]: fix oss fuzz 32643, Config validation of lb_subset_config for server fuzz test #16582
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Commit Message: In server fuzz test, misconfiguration of subset_selectors will cause the test to failed.(https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32643) When we set single_host_per_subset to be true and specify more than 1 key, envoy will throw unexpected exception. Since the correct configuration of subset_selectors is well documented, we can reject such misconfiguration to prevent the fuzz test failure.
Additional Description:
Risk Level: low
Testing: add config validation to server fuzz test.
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
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