test allocation limits

tests/ImageSharp.Tests/Memory/Allocators/UniformUnmanagedPoolMemoryAllocatorTests.cs

@@ -418,4 +418,28 @@ public class UniformUnmanagedPoolMemoryAllocatorTests
             _ = MemoryAllocator.Create();
         }
     }
+
+    [Fact]
+    public void Allocate_OverLimit_ThrowsInvalidMemoryOperationException()
+    {
+        MemoryAllocator allocator = MemoryAllocator.Create(new MemoryAllocatorOptions()
+        {
+            AllocationLimitMegabytes = 4
+        });
+        const int oneMb = 1 << 20;
+        allocator.Allocate<byte>(4 * oneMb).Dispose(); // Should work
+        Assert.Throws<InvalidMemoryOperationException>(() => allocator.Allocate<byte>(5 * oneMb));
+    }
+
+    [Fact]
+    public void AllocateGroup_OverLimit_ThrowsInvalidMemoryOperationException()
+    {
+        MemoryAllocator allocator = MemoryAllocator.Create(new MemoryAllocatorOptions()
+        {
+            AllocationLimitMegabytes = 4
+        });
+        const int oneMb = 1 << 20;
+        allocator.AllocateGroup<byte>(4 * oneMb, 1024).Dispose(); // Should work
+        Assert.Throws<InvalidMemoryOperationException>(() => allocator.AllocateGroup<byte>(5 * oneMb, 1024));
+    }
 }