Answer to myself and maybe others:

The tests do not want to compare the instances, as they (hopefully) are always different.
Take care of this early and all will be fine :)

Maybe this was the same error I'm running into:

Test.assert_equals(Vector.new(1,2,3), Vector.new([1,2,3]))

yields

#<Vector:0x0000559eb6bc3b08 @x=1, @y=2, @z=3>, instead got: #<Vector:0x0000559eb6bc3f68 @x=1, @y=2, @z=3>

in my implementation, although the initialization itselfs seems to do, what it is supposed to.

Without knowing what you are doing it's difficult to say what you are doing wrong :)