Fix proc_net_test_native for native tests.

"DefaultValueEqZero" is only valid if the test is in a
sandbox. Our CI VMs often have "/proc/sys/net/ipv4/ip_forward" set
to 1.

PiperOrigin-RevId: 332910859

test/syscalls/linux/proc_net.cc

@@ -522,6 +522,9 @@ TEST(ProcSysNetIpv4IpForward, Exists) {
 }
 
 TEST(ProcSysNetIpv4IpForward, DefaultValueEqZero) {
+  // Test is only valid in sandbox. Not hermetic in native tests
+  // running on a arbitrary machine.
+  SKIP_IF(!IsRunningOnGvisor());
   auto const fd = ASSERT_NO_ERRNO_AND_VALUE(Open(kIpForward, O_RDONLY));
 
   char buf = 101;