Simply initialize `point` with something and then check it stayed the same.

```cpp
Eigen::Vector3f point (1, 2, 3);
EXPECT_FALSE(threeplanesWithPointIntersection(plane_a, plane_b, plane_c, point, 1e-6));
EXPECT_FLOAT_EQ (1, point (0));
EXPECT_FLOAT_EQ (2, point (1));
EXPECT_FLOAT_EQ (3, point (2));
```