add admin test coverage (#18582)
add admin test coverage (#18582)

Signed-off-by: chaoqin-li1123 <7aa9e142072b95ee36c6bce3c990361972ed32b6@google.com>
envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,lyft/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy