[testing] Add StopListening helper and BidiStream handler to XDS e2e test lib (#35861)

As title. Pulling these additions out from a larger change.

Related: cl/563857636

Closes #35861

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35861 from apolcyn:xds_test_changes a67f64d93c
PiperOrigin-RevId: 607728140

test/cpp/end2end/xds/xds_end2end_test_lib.cc

@@ -171,6 +171,16 @@ void XdsEnd2endTest::ServerThread::StopListeningAndSendGoaways() {
   gpr_log(GPR_INFO, "%s done sending GOAWAYs", Type());
 }
 
+void XdsEnd2endTest::ServerThread::StopListening() {
+  gpr_log(GPR_INFO, "%s about to stop listening", Type());
+  {
+    grpc_core::ExecCtx exec_ctx;
+    auto* server = grpc_core::Server::FromC(server_->c_server());
+    server->StopListening();
+  }
+  gpr_log(GPR_INFO, "%s stopped listening", Type());
+}
+
 void XdsEnd2endTest::ServerThread::Serve(grpc_core::Mutex* mu,
                                          grpc_core::CondVar* cond) {
   // We need to acquire the lock here in order to prevent the notify_one

test/cpp/end2end/xds/xds_end2end_test_lib.h

@@ -259,6 +259,8 @@ class XdsEnd2endTest : public ::testing::TestWithParam<XdsTestType>,
       allow_put_requests_ = allow_put_requests;
     }
 
+    void StopListening();
+
     void StopListeningAndSendGoaways();
 
    private: