Add tests for sockopt::IpMulticastTtl #2073

On Linux and Android, sockopt::IpMulticastTtl can be set for both IPv4
and IPv6 sockets, but not on all platforms.

test/sys/test_sockopt.rs

@@ -330,6 +330,34 @@ fn test_ttl_opts() {
         .expect("setting ipv6ttl on an inet6 socket should succeed");
 }
 
+#[test]
+#[cfg(any(target_os = "android", target_os = "freebsd", target_os = "linux"))]
+fn test_multicast_ttl_opts_ipv4() {
+    let fd4 = socket(
+        AddressFamily::Inet,
+        SockType::Datagram,
+        SockFlag::empty(),
+        None,
+    )
+    .unwrap();
+    setsockopt(&fd4, sockopt::IpMulticastTtl, &2)
+        .expect("setting ipmulticastttl on an inet socket should succeed");
+}
+
+#[test]
+#[cfg(any(target_os = "android", target_os = "linux"))]
+fn test_multicast_ttl_opts_ipv6() {
+    let fd6 = socket(
+        AddressFamily::Inet6,
+        SockType::Datagram,
+        SockFlag::empty(),
+        None,
+    )
+    .unwrap();
+    setsockopt(&fd6, sockopt::IpMulticastTtl, &2)
+        .expect("setting ipmulticastttl on an inet6 socket should succeed");
+}
+
 #[test]
 #[cfg(any(target_os = "ios", target_os = "macos"))]
 fn test_dontfrag_opts() {