Add new generic collections unit tests (#180)

***NO_CI***

Author: josesimoes

Tests/GenericCollections/ListTests.cs

@@ -11,6 +11,16 @@ namespace GenericCollections
     [TestClass]
     public class ListTests
     {
+        [Cleanup]
+        public void CleanupTests()
+        {
+            // StaticListHolder is shared static state (see nanoframework/Home#1821
+            // repro below); clear it so List_StaticField_* tests start empty on
+            // every run/retry, even if a prior test's assertions failed.
+            StaticListHolder.Numbers.Clear();
+            StaticListHolder.Items.Clear();
+        }
+
         [TestMethod]
         public void List_Constructor_Default()
         {
@@ -1203,6 +1213,32 @@ public void List_GenericMethod_NewObj_InsertAndIterate()
             Assert.AreEqual(4, result[3]);
         }
 
+        [TestMethod]
+        public void List_StaticField_ValueType_Add()
+        {
+            // Reproduces nanoframework/Home#1821: a List<T> held in a static readonly
+            // field initialized via a field initializer (i.e. constructed from the
+            // declaring type's own .cctor), then Add()'d to from another method.
+            StaticListHolder.Numbers.Add(1);
+            StaticListHolder.Numbers.Add(2);
+
+            Assert.AreEqual(2, StaticListHolder.Numbers.Count);
+            Assert.AreEqual(1, StaticListHolder.Numbers[0]);
+            Assert.AreEqual(2, StaticListHolder.Numbers[1]);
+        }
+
+        [TestMethod]
+        public void List_StaticField_ReferenceType_Add()
+        {
+            // Reproduces nanoframework/Home#1821 with a reference-type element,
+            // mirroring the reported List<Ds18b20> Sensors static field.
+            StaticListHolder.Items.Add(new DummyClass(1, "One"));
+
+            Assert.AreEqual(1, StaticListHolder.Items.Count);
+            Assert.AreEqual(1, StaticListHolder.Items[0].Id);
+            Assert.AreEqual("One", StaticListHolder.Items[0].Name);
+        }
+
         // Generic helper methods that exercise NEWOBJ with MVAR TypeSpecs.
         // Each method creates a new List<T> from within a generic method body,
         // producing IL with MethodRef tokens whose owner TypeSpec is List<!!0>.
@@ -1316,6 +1352,14 @@ public DummyClass(int id, string name)
         }
     }
 
+    // Field initializers below run as part of this type's own .cctor, matching the
+    // pattern reported in nanoframework/Home#1821 (`private static readonly List<T> ... = new List<T>();`).
+    internal static class StaticListHolder
+    {
+        internal static readonly List<int> Numbers = new List<int>();
+        internal static readonly List<DummyClass> Items = new List<DummyClass>();
+    }
+
     internal struct DummyStruct
     {
         public int Id { get; set; }