fix: Silence non-nullable warning in pool tests.

Author: shanecelis

src/NetMQ.Tests/BufferPoolTests.cs

@@ -10,7 +10,10 @@ public abstract class BufferPoolTests
     {
         protected int maxBufferSize = 2;
         protected long maxBufferPoolSize = 100L;
+        // Silence the non-nullable warning.
+#pragma warning disable 8618
         protected IBufferPool pool;
+#pragma warning restore 8618
 
         [Theory]
         [InlineData(500)]