Add manual test for CG converence

Also add MJTOL_SCALE to fixture to allow tests to be run with zero tolerance. This is useful when assesing the impact of code changes (A/B comparison of failure values)

PiperOrigin-RevId: 924219083
Change-Id: Ifdd09ac850904ca8dd79179930ce738a4b37d284

test/engine/engine_util_spatial_test.cc

@@ -121,7 +121,7 @@ TEST_F(RotVecQuatTest, TestEquivalence) {
       {1, 0, 0}, {0, 1, 0}, {0, 0, 1}, {-0.5, 1, -0.5}, {1.22, -2.33, 3.44}};
   // List of angles to rotate by, in degrees
   mjtNum angles[6] = {0.0, 1e-8, 31, 47, 181, 271};
-  static constexpr mjtNum eps = MjTol(1e-15, 1e-5);
+  static const mjtNum eps = MjTol(1e-15, 1e-5);
   for (auto vec : vecs) {
     // Unit-normalize the vector
     mju_normalize3(vec);