[mlir] Adjust AttributeTest to show skip behavior.

[akuegel]

Summary

walk() skips identical sub-attributes. Adjust the unit test to show
this.

mlir/unittests/IR/AttributeTest.cpp

 //===----------------------------------------------------------------------===//
 
 TEST(SubElementTest, Nested) {
   MLIRContext context;
   Builder builder(&context);
 
   BoolAttr trueAttr = builder.getBoolAttr(true);
   BoolAttr falseAttr = builder.getBoolAttr(false);
-  ArrayAttr boolArrayAttr = builder.getArrayAttr({trueAttr, falseAttr});
+  ArrayAttr boolArrayAttr =
+      builder.getArrayAttr({trueAttr, falseAttr, trueAttr});
   StringAttr strAttr = builder.getStringAttr("array");
   DictionaryAttr dictAttr =
       builder.getDictionaryAttr(builder.getNamedAttr(strAttr, boolArrayAttr));
 
   SmallVector<Attribute> subAttrs;
   dictAttr.walk([&](Attribute attr) { subAttrs.push_back(attr); });
+  // Note that trueAttr appears only once, identical subattributes are skipped.
   EXPECT_EQ(llvm::ArrayRef(subAttrs),
             ArrayRef<Attribute>(
                 {strAttr, trueAttr, falseAttr, boolArrayAttr, dictAttr}));
 }
 } // namespace