lib/logger, lib/model: Correct printf format strings in tests

This is a build (test) failure in Go 1.10.

lib/logger/logger_test.go

@@ -147,7 +147,7 @@ func TestRecorder(t *testing.T) {
 		t.Fatalf("unexpected line count: %d", len(lines))
 	}
 	if lines[0].Message != "hah" {
-		t.Errorf("incorrect line: %s", lines[0])
+		t.Errorf("incorrect line: %s", lines[0].Message)
 	}
 
 }