tests/matrix: Use a bigger angle

Ten degrees creates a small difference between values which could be
misinterpreted as a floating point fluctuation.
matrix: Multiplication is non-commutative

The order to multiply two matrices is important.

When transforming a matrix with a rotation, translation, or scale, we
need to multiply the transformation matrix with the given matrix, not
the other way around.

Issue #9.
tests/matrix: Ensure that rotations are additive

Rotating by 45° → 15° is equivalent to rotating by 60°.