Add mj_copyBack for copying real-valued arrays from mjModel back to mjSpec.

Also add `SaveAndReadXML` function to test fixture using `mjSpec` as input.

PiperOrigin-RevId: 765393821
Change-Id: Ic257addd2fc89678fc11b226c168c077626cc51e

test/fixture.h

@@ -102,6 +102,9 @@ mjModel* LoadModelFromPath(const char* model_path);
 // Returns a string loaded from first saving the model given an input.
 std::string SaveAndReadXml(const mjModel* model);
 
+// Returns a string loaded from first saving the spec given an input.
+std::string SaveAndReadXml(const mjSpec* spec);
+
 // Adds control noise.
 std::vector<mjtNum> GetCtrlNoise(const mjModel* m, int nsteps,
                                  mjtNum ctrlnoise = 0.01);