Use std::abs instead of mju_abs to compare integerst in fixture.cc.

PiperOrigin-RevId: 659534808
Change-Id: I4eb9f72078f2d7224bcfd3b537e4053d220750c8

test/fixture.cc

@@ -17,6 +17,7 @@
 #include <array>
 #include <cerrno>
 #include <cstdio>
+#include <cstdlib>
 #include <cstring>
 #include <filesystem>  // NOLINT
 #include <fstream>
@@ -218,15 +219,16 @@ mjtNum CompareModel(const mjModel* m1, const mjModel* m2,
   // (needed in MJMODEL_POINTERS)
   MJMODEL_POINTERS_PREAMBLE(m1);
 
-  // compare ints, exclude nbuffer because it hides the actual difference
-  #define X(name) \
-    if (strncmp(#name, "nbuffer", 7)) {                            \
-      if (m1->name != m2->name) {                                  \
-        maxdif = mju_abs(m1->name - m2->name); field = #name;      \
-      }                                                            \
-    }
-    MJMODEL_INTS
-  #undef X
+// compare ints, exclude nbuffer because it hides the actual difference
+#define X(name)                                           \
+  if constexpr (std::string_view(#name) != "nbuffer") {   \
+    if (m1->name != m2->name) {                           \
+      maxdif = std::abs((long)m1->name - (long)m2->name); \
+      field = #name;                                      \
+    }                                                     \
+  }
+  MJMODEL_INTS
+#undef X
   if (maxdif > 0) return maxdif;
 
   // compare arrays, apart from bvh-related ones, as those are sensitive to