Add test

Author: hadashiA

tests/MemoryPack.Tests/GeneratorDiagnosticsTest.cs

@@ -653,10 +653,41 @@ public void MEMPACK040_SuppressDefaultInitializationMustBeSettable()
 public partial class Tester
 {
     [SuppressDefaultInitialization]
-    public required int I1 { get; init; }
+    public required int I1 { get; set; }
 }
 
 """);
+
+        Compile(40, """
+using MemoryPack;
+
+[MemoryPackable]
+public partial class Tester
+{
+    [SuppressDefaultInitialization]
+    public int I1 { get; init; }
+}
+
+""");
+
+        Compile(40, """
+using MemoryPack;
+
+[MemoryPackable]
+public partial class Tester
+{
+    [SuppressDefaultInitialization]
+    public readonly int I1;
+
+    [MemoryPackConstructor]
+    public Tester(int i1)
+    {
+        I1 = i1;
+    }
+}
+
+""");
+
     }
 }