[NFC][scudo] Fix sign-compare warning in test

GitOrigin-RevId: b850798f11d3428d6edc22ad63b9cabe2e6baec1
diff --git a/tests/memtag_test.cpp b/tests/memtag_test.cpp
index 13c853f..8726fc2 100644
--- a/tests/memtag_test.cpp
+++ b/tests/memtag_test.cpp
@@ -74,7 +74,7 @@
   // expected range.
   for (u64 Top = 0; Top < 0x100; ++Top)
     Tags = Tags | (1u << extractTag(Addr | (Top << 56)));
-  EXPECT_EQ(0xffff, Tags);
+  EXPECT_EQ(0xffffull, Tags);
 }
 
 TEST_F(MemtagTest, AddFixedTag) {