unit test remove first node (#440)

Author: bitfaster

BitFaster.Caching.UnitTests/Lfu/LfuNodeListTests.cs

@@ -1,11 +1,22 @@
-using BitFaster.Caching.Lfu;
+using System;
+using BitFaster.Caching.Lfu;
 using FluentAssertions;
 using Xunit;
 
 namespace BitFaster.Caching.UnitTests.Lfu
 {
     public class LfuNodeListTests
     {
+#if DEBUG
+        [Fact]
+        public void WhenEmptyRemoveFirstThrows()
+        {
+            var list = new LfuNodeList<int, int>();
+            Action remove = () => { list.RemoveFirst(); };
+            remove.Should().Throw<InvalidOperationException>();
+        }
+#endif
+
         [Fact]
         public void WhenPreviousNullLastReturnsNull()
         {