test/bufferlist: For root, don't do permission operation for read_file

test/bufferlist: For root, don't do permission operation for read_file
case.

For root user, it meet those error:
    test/bufferlist.cc:1880: Failure
    Value of: bl.read_file("testfile", &error)
      Actual: 0
    Expected: -13
    test/bufferlist.cc:1884: Failure
    Value of: bl.length()
      Actual: 8
    Expected: (unsigned)4
    Which is: 4
    test/bufferlist.cc:1886: Failure
    Value of: actual
      Actual: "ABC
    ABC
    "
    Expected: "ABC\n"
    Which is: "ABC

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