[unit-test] add a common function to dump buffer content (#4367)

This commit updates `test_utils.hpp/cpp` to include a common helper
function `DumpBuffer` to print the content of a buffer (as hex and char string)
to screen. It also removes unused helper functions and use of STL header
files and types.

tests/unit/test_spinel_encoder.cpp

@@ -26,13 +26,11 @@
  *  POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <ctype.h>
-
 #include "common/code_utils.hpp"
 #include "common/instance.hpp"
 #include "ncp/spinel_encoder.hpp"
 
-#include "test_util.h"
+#include "test_util.hpp"
 
 namespace ot {
 namespace Ncp {
@@ -42,47 +40,6 @@ enum
     kTestBufferSize = 800,
 };
 
-// Dump the buffer content to screen.
-void DumpBuffer(const char *aTextMessage, uint8_t *aBuffer, uint16_t aBufferLength)
-{
-    enum
-    {
-        kBytesPerLine = 32, // Number of bytes per line.
-    };
-
-    char     charBuff[kBytesPerLine + 1];
-    uint16_t counter;
-    uint8_t  byte;
-
-    printf("\n%s - len = %u\n    ", aTextMessage, aBufferLength);
-
-    counter = 0;
-
-    while (aBufferLength--)
-    {
-        byte = *aBuffer++;
-        printf("%02X ", byte);
-        charBuff[counter] = isprint(byte) ? static_cast<char>(byte) : '.';
-        counter++;
-
-        if (counter == kBytesPerLine)
-        {
-            charBuff[counter] = 0;
-            printf("    %s\n    ", charBuff);
-            counter = 0;
-        }
-    }
-
-    charBuff[counter] = 0;
-
-    while (counter++ < kBytesPerLine)
-    {
-        printf("   ");
-    }
-
-    printf("    %s\n", charBuff);
-}
-
 otError ReadFrame(NcpFrameBuffer &aNcpBuffer, uint8_t *aFrame, uint16_t &aFrameLen)
 {
     otError error = OT_ERROR_NONE;