Refactor error assertion in S/MIME signing test

Updated the test to check if the error message contains the expected substring instead of requiring an exact match. This makes the test more resilient to minor changes in error formatting.

msg_test.go

@@ -5934,9 +5934,9 @@ func TestMsg_WriteTo(t *testing.T) {
 		if err == nil {
 			t.Error("expected WriteTo with invalid S/MIME private key to fail")
 		}
-		expErr := "failed to read from rand.Reader: broken reader"
-		if !strings.EqualFold(err.Error(), expErr) {
-			t.Errorf("expected S/MIME signing error to be: %s, got: %s", expErr, err)
+		expErr := "broken reader"
+		if !strings.Contains(err.Error(), expErr) {
+			t.Errorf("expected S/MIME signing error to contain: %q, got: %s", expErr, err)
 		}
 	})
 }