Fix warning in test_strunescape

Fix warning in test_strunescape

src/utils/common/common_test.c:254:3: warning: ‘strncpy’ output truncated before terminating nul copying 16 bytes from a string of the same length [-Wstringop-truncation]
  254 |   strncpy(buffer, "\\tbackslash end\\", sizeof(buffer));
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  CCLD     test_common