Fix StructuralSparseArray Clear function and add basic test

src/Arch.Tests/StructuralSparseArrayTest.cs

@@ -0,0 +1,57 @@
+using Arch.Buffer;
+
+namespace Arch.Tests;
+
+[TestFixture]
+public sealed partial class StructuralSparseArrayTest
+{
+    private static void TestEquivalent(StructuralSparseArray test, HashSet<int> control)
+    {
+        // Brute force test every index
+        for (int i = 0; i < 128; i++)
+        {
+            bool contains = control.Contains(i);
+            Assert.That(test.Contains(i), Is.EqualTo(contains));
+        }
+    }
+
+    [Test]
+    public void ClearAndAccessMany()
+    {
+        var test = new StructuralSparseArray(new(1, 0));
+        var control = new HashSet<int>();
+
+        for (int i = 0; i < 10; i++)
+        {
+            control.Add(52 + i);
+            test.Add(52 + i);
+
+            TestEquivalent(test, control);
+
+            control.Add(3 + i);
+            test.Add(3 + i);
+
+            TestEquivalent(test, control);
+
+            control.Clear();
+            test.Clear();
+
+            TestEquivalent(test, control);
+
+            control.Add(3);
+            test.Add(3);
+
+            TestEquivalent(test, control);
+
+            control.Add(3);
+            test.Add(3);
+
+            TestEquivalent(test, control);
+
+            control.Clear();
+            test.Clear();
+
+            TestEquivalent(test, control);
+        }
+    }
+}

src/Arch/Buffer/StructuralSparseSet.cs

@@ -103,7 +103,7 @@ public bool Contains(int index)
     /// </summary>
     public void Clear()
     {
-        Array.Fill(Entities, -1, 0, Size);
+        Array.Fill(Entities, -1, 0, Entities.Length);
         Size = 0;
     }
 }