Improve MatMulNBits test (#19378)

### Description
The test creates millions of threads. This change is to avoid that by
using an existing thread pool.


### Motivation and Context

onnxruntime/test/contrib_ops/matmul_4bits_test.cc

@@ -14,29 +14,31 @@
 #include "test/optimizer/graph_transform_test_builder.h"
 #include "test/providers/provider_test_utils.h"
 #include "test/util/include/default_providers.h"
+#include "core/session/onnxruntime_cxx_api.h"
+#include "core/session/ort_env.h"
 #include "core/util/qmath.h"
 
 #include <chrono>
 #include <random>
 
 #include "gtest/gtest.h"
 #include "gmock/gmock.h"
+extern std::unique_ptr<Ort::Env> ort_env;
 
 namespace onnxruntime {
+
 namespace test {
 
 static constexpr int QBits = 4;
-
 void QuantizeDequantize(std::vector<float>& raw_vals,
                         std::vector<uint8_t>& quant_vals,
                         std::vector<float>& scales,
                         std::vector<uint8_t>* zp,
                         int32_t N,
                         int32_t K,
                         int32_t block_size) {
-  OrtThreadPoolParams to;
-  auto tp = concurrency::CreateThreadPool(&onnxruntime::Env::Default(), to,
-                                          concurrency::ThreadPoolType::INTRA_OP);
+  auto& ortenv = **ort_env.get();
+  onnxruntime::concurrency::ThreadPool* tp = ortenv.GetEnvironment().GetIntraOpThreadPool();
 
   MlasQuantizeBlockwise<float, 4>(
       quant_vals.data(),
@@ -48,7 +50,7 @@ void QuantizeDequantize(std::vector<float>& raw_vals,
       K,
       N,
       N,
-      tp.get());
+      tp);
 
   // Note that input1_f_vals is NxK after dequant
   MlasDequantizeBlockwise<float, 4>(
@@ -60,7 +62,7 @@ void QuantizeDequantize(std::vector<float>& raw_vals,
       true,                                  // columnwise quantization
       K,                                     // number of rows
       N,                                     // number of columns
-      tp.get());
+      tp);
 }
 
 void RunTest(int64_t M, int64_t N, int64_t K, int64_t block_size, int64_t accuracy_level,