test_session.c: fix warning for taken absolute value of unsigned int
test_session.c: fix warning for taken absolute value of unsigned int

This change replaces the call of abs(v - ref) to avoid a compiler
warning for -Wabsolute-value:

test_session.c:95:11: warning: taking the absolute value of unsigned type 'unsigned int' has no effect
      [-Wabsolute-value]
  return (abs(v - ref) / 100.0) <= FP_ERR_THRESHOLD;

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap