Add RecyclablePriorityQueue with tests

Recyclable.CollectionsTests/RecyclablePriorityQueueTests.cs

@@ -0,0 +1,56 @@
+using FluentAssertions;
+using Recyclable.Collections;
+
+namespace Recyclable.CollectionsTests
+{
+    public class RecyclablePriorityQueueTests
+    {
+        private static readonly int[] _testData = new[] { 4, 1, 3, 2, 5 };
+
+        [Fact]
+        public void DequeueShouldReturnItemsInSortedOrder()
+        {
+            using var queue = new RecyclablePriorityQueue<int>();
+            foreach (var item in _testData)
+            {
+                queue.Enqueue(item);
+            }
+
+            var result = new List<int>();
+            while (queue.LongCount > 0)
+            {
+                result.Add(queue.Dequeue());
+            }
+
+            _ = result.Should().BeInAscendingOrder();
+        }
+
+        [Fact]
+        public void ShouldBeEmptyWhenNotInitialized()
+        {
+            using var queue = new RecyclablePriorityQueue<int>();
+            _ = queue.Should().BeEmpty();
+            _ = queue.LongCount.Should().Be(0);
+        }
+
+        [Fact]
+        public void ShouldBeEmptyAfterClear()
+        {
+            using var queue = new RecyclablePriorityQueue<int>();
+            foreach (var item in _testData)
+            {
+                queue.Enqueue(item);
+            }
+
+            queue.Clear();
+            _ = queue.Should().BeEmpty();
+        }
+
+        [Fact]
+        public void DequeueShouldThrowWhenNoElements()
+        {
+            using var queue = new RecyclablePriorityQueue<int>();
+            _ = Assert.Throws<ArgumentOutOfRangeException>(() => queue.Dequeue());
+        }
+    }
+}