[TEST] Update gravity test to disable scalar gradient computation

Modified the approved test file to set `compute_scalar_gradient` to `false`, ensuring consistency with test expectations and configuration.

Author: Leguark

test/test_modules/_geophysics_TO_UPDATE/test_gravity.test_gravity.verify/2-layers.approved.txt

@@ -162,7 +162,7 @@
             "mesh_extraction_masking_options": 3,
             "mesh_extraction_fancy": true,
             "evaluation_chunk_size": 500000,
-            "compute_scalar_gradient": true,
+            "compute_scalar_gradient": false,
             "verbose": false
         },
         "debug": true,