Ensure last XML is freed after tests.

This fixes undetected memory leaks in tests calling mj_loadXML.

Additionally, a specific test for `mj_freeLastXML` is introduced.

PiperOrigin-RevId: 764294041
Change-Id: I8b5a5683d8431ed920cdd52b06ad3515d8e19e14

test/fixture.h

@@ -53,6 +53,9 @@ class MujocoErrorTestGuard {
 // By default, any MuJoCo operation which triggers a warning or error will
 // trigger a test failure.
 class MujocoTest : public ::testing::Test {
+ public:
+  ~MujocoTest() { mj_freeLastXML(); }
+
  private:
   MujocoErrorTestGuard error_guard;
 };