mobile: Fix flaky ClientIntegrationTest (envoyproxy#36077)

This PR removes the test expectation that the `end_stream` always
contains 0 bytes since this behavior is not guaranteed causing test
flakiness. This is similar to
envoyproxy#34252. See also
envoyproxy/envoy-mobile#1393.

Fixes envoyproxy#36074.

Risk Level: low (test only)
Testing: `bazel test //test/common/integration:client_integration_test
--runs_per_test=200`
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

---------

Signed-off-by: Fredy Wijaya <[email protected]>

mobile/test/common/integration/client_integration_test.cc

@@ -206,13 +206,7 @@ void ClientIntegrationTest::basicTest() {
                                    std::to_string(request_data.length()));
 
   EnvoyStreamCallbacks stream_callbacks = createDefaultStreamCallbacks();
-  stream_callbacks.on_data_ = [this](const Buffer::Instance& buffer, uint64_t length,
-                                     bool end_stream, envoy_stream_intel) {
-    if (end_stream) {
-      std::string response_body(length, ' ');
-      buffer.copyOut(0, length, response_body.data());
-      EXPECT_EQ(response_body, "");
-    }
+  stream_callbacks.on_data_ = [this](const Buffer::Instance&, uint64_t, bool, envoy_stream_intel) {
     cc_.on_data_calls_++;
   };