clean up test code and make plane defined more clear

Plane is defined as: Ax+By+Cz+D=0, same as: nx*px+ny*py+nz*pz+d=0, where (nx, ny, nz) is the plane normal, (px, py, pz) is the point lay on the plane. so d = -dot(n, p) test.lua are split into more test file, which are in the **_test_** folder