x/net/ipv4: fix full stack test cases

Readjusts ICMP test cases on older darwin kernels
Also simplifies error logs.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/178740043
diff --git a/ipv4/multicastsockopt_test.go b/ipv4/multicastsockopt_test.go
index 56df5be..7235d68 100644
--- a/ipv4/multicastsockopt_test.go
+++ b/ipv4/multicastsockopt_test.go
@@ -120,7 +120,7 @@
 		t.Error(err)
 		return
 	} else if v != ttl {
-		t.Errorf("got unexpected multicast ttl %v; expected %v", v, ttl)
+		t.Errorf("got %v; want %v", v, ttl)
 		return
 	}
 
@@ -133,7 +133,7 @@
 			t.Error(err)
 			return
 		} else if v != toggle {
-			t.Errorf("got unexpected multicast loopback %v; expected %v", v, toggle)
+			t.Errorf("got %v; want %v", v, toggle)
 			return
 		}
 	}