In cxxtest, TS_ASSERT_DIFFERS(a, b) tests !(a == b) and not (a != b).
Therefore, to test a::operator!=(b), we must avoid TS_ASSERT_DIFFERS()
(that will test !a::operator==(b)) and write instead TS_ASSERT(a != b).
The new class Direction will be used to represent the direction of a
calibration request.
Update posexcept.hh to add new exception bad_direction, used in class
Direction.