Fix detournavigator test
Pass min/max to the btHeightfieldTerrainShape
constructor.
This is required and only happened to work because
shape.processAllTriangles
did not check the Z axis.
This may change in the future, e.g. see https://github.com/bulletphysics/bullet3/pull/3287
Sets the min/maxValue so that they encompass the values but also so that
(maxValue + minValue) / 2
is zero. This is because bullet always sets
heightfield origin Z to the midpoint between min and max height.
Ensuring that the midpoint is zero means we don't have to change the test fixtures.