fix compiler warnings for CPP tests (#6124)

tests/cpp_tests/test_byte_buffer.cpp

@@ -30,7 +30,7 @@ TEST(ByteBuffer, JustWorks) {
   EXPECT_EQ(cumulativeSize, buffer->GetSize());
   int16_t serializedInt16 = 0;
   char* int16Ptr = reinterpret_cast<char*>(&serializedInt16);
-  for (int i = 0; i < sizeof(int16_t); i++) {
+  for (unsigned int i = 0; i < sizeof(int16_t); i++) {
     int16Ptr[i] = buffer->GetAt(cumulativeSize - (sizeof(int16_t) - i));
   }
   EXPECT_EQ(int16Val, serializedInt16);
@@ -41,7 +41,7 @@ TEST(ByteBuffer, JustWorks) {
   EXPECT_EQ(cumulativeSize, buffer->GetSize());
   int64_t serializedInt64 = 0;
   char* int64Ptr = reinterpret_cast<char*>(&serializedInt64);
-  for (int i = 0; i < sizeof(int64_t); i++) {
+  for (unsigned int i = 0; i < sizeof(int64_t); i++) {
     int64Ptr[i] = buffer->GetAt(cumulativeSize - (sizeof(int64_t) - i));
   }
   EXPECT_EQ(int64Val, serializedInt64);
@@ -52,7 +52,7 @@ TEST(ByteBuffer, JustWorks) {
   EXPECT_EQ(cumulativeSize, buffer->GetSize());
   double serializedDouble = 0;
   char* doublePtr = reinterpret_cast<char*>(&serializedDouble);
-  for (int i = 0; i < sizeof(double); i++) {
+  for (unsigned int i = 0; i < sizeof(double); i++) {
     doublePtr[i] = buffer->GetAt(cumulativeSize - (sizeof(double) - i));
   }
   EXPECT_EQ(doubleVal, serializedDouble);

tests/cpp_tests/testutils.cpp

@@ -265,7 +265,7 @@ namespace LightGBM {
                      groups_ptr,
                      thread_count,
                      t);
-      threads.push_back(move(th));
+      threads.push_back(std::move(th));
     }
 
     for (auto& t : threads) t.join();