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/user/user_objects_test.cc

@@ -31,7 +31,7 @@
 namespace mujoco {
 namespace {
 
-constexpr double kInertiaTol = MjTol(1e-6, 1e-6);
+const double kInertiaTol = MjTol(1e-6, 1e-6);
 
 using std::string;
 using ::testing::ElementsAre;