chore(ci): increase server start up wait time in interop test (#2473)

## Motivation

The interop test in post-merge CI step is flaky due to the wait time
between gRPC server start up and test request being too low (1 second).
Example failure:
https://github.com/hyperium/tonic/actions/runs/20730918623/job/59518404139

## Solution

This PR is a simple improvement to reduce the chance of failure to
unblock CI. For a more fundamental fix to the root cause, we can plan
accordingly as we iterate `grpc-rust`.

Author: YutaoMa

interop/test.sh

@@ -60,7 +60,7 @@ cleanup() {
 # regardless of why (errors, SIGTERM, etc).
 trap cleanup EXIT
 
-sleep 1
+sleep 3
 
 ./target/debug/client --codec=prost --test_case="${JOINED_TEST_CASES}" "${ARG}"
 
@@ -81,7 +81,7 @@ for CODEC in "${CODECS[@]}"; do
     SERVER_PID=$!
     echo ":; started tonic test server with the ${CODEC} codec."
 
-    sleep 1
+    sleep 3
 
     ./target/debug/client --codec=prost --test_case="${JOINED_TEST_CASES}" "${ARG}"