Improve diagnostics for int128 test failures

Author: kg

src/tests/Interop/PInvoke/Int128/Int128Test.cs

@@ -10,13 +10,19 @@ public struct StructJustInt128
 {
     public StructJustInt128(Int128 val) { value = val; }
     public Int128 value;
+
+    public override string ToString () =>
+        $"StructJustInt128(value={value:X32})";
 }
 
 public struct StructWithInt128
 {
     public StructWithInt128(Int128 val) { value = val; messUpPadding = 0x10; }
     public byte messUpPadding;
     public Int128 value;
+
+    public override string ToString () =>
+        $"StructWithInt128(messUpPadding={messUpPadding}, value={value:X32})";
 }
 
 unsafe partial class Int128Native
@@ -111,6 +117,7 @@ public static void TestInt128FieldLayout()
         StructWithInt128 rhs = new StructWithInt128(new Int128(13, 14));
 
         Int128Native.AddStructWithInt128_ByRef(ref lhs, ref rhs);
+        // Interpreter-FIXME: Incorrect result
         Assert.Equal(new StructWithInt128(new Int128(24, 26)), lhs);
 
         Int128 value2;