test/unit/box: fix failing tests

Apparently, initializer list constructors are preferred over
constructors, so the position was initialized only in the first element.
Fix it by introducing a unit vector which is scaled accordingly, this
also makes the code more readable.

Fixes #341563
fixed_vector: test construction with initialiser lists

Initialiser lists caused the problem reported in bug #341563.
box test: shorten syntax of initialiser lists

The explicit "vector_type" is only needed in ternary expressions. The
unit vector is more transparently constructed using initialisation by
a scalar value.