test: fix compile warning in bufferlist.cc

test/bufferlist.cc: In member function ‘virtual void
Buffer_constructors_Test::TestBody()’:
test/bufferlist.cc:154:36: warning: ignoring return value of ‘int
system(const char*)’, declared with attribute warn_unused_result
[-Wunused-result]
     ::system("echo ABC > testfile");
                                    ^
test/bufferlist.cc: In member function ‘virtual void
TestRawPipe::SetUp()’:
test/bufferlist.cc:182:36: warning: ignoring return value of ‘int
system(const char*)’, declared with attribute warn_unused_result
[-Wunused-result]
     ::system("echo ABC > testfile");
                                    ^
test/bufferlist.cc: In member function ‘virtual void
BufferList_read_file_Test::TestBody()’:
test/bufferlist.cc:1768:53: warning: ignoring return value of ‘int
system(const char*)’, declared with attribute warn_unused_result
[-Wunused-result]
   ::system("echo ABC > testfile ; chmod 0 testfile");
                                                     ^
test/bufferlist.cc:1770:32: warning: ignoring return value of ‘int
system(const char*)’, declared with attribute warn_unused_result
[-Wunused-result]
   ::system("chmod +r testfile");
                                ^
test/bufferlist.cc: In member function ‘virtual void
BufferList_read_fd_Test::TestBody()’:
test/bufferlist.cc:1781:34: warning: ignoring return value of ‘int
system(const char*)’, declared with attribute warn_unused_result
[-Wunused-result]
   ::system("echo ABC > testfile");

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>