Fix NUnit analyzer errors: Replace Is.EqualTo(0) with Is.Zero

Co-authored-by: glennawatson <5834289+glennawatson@users.noreply.github.com>

Author: Copilot

src/ReactiveUI.Extensions.Tests/ReactiveExtensionsTests.cs

@@ -99,7 +99,7 @@ public void SubscribeSynchronus_RunsWithAsyncTasksInSubscriptions()
         }
 
         // Then
-        Assert.That(result, Is.EqualTo(0));
+        Assert.That(result, Is.Zero);
     }
 
     /// <summary>
@@ -144,7 +144,7 @@ public void SyncronizeAsync_RunsWithAsyncTasksInSubscriptions()
         }
 
         // Then
-        Assert.That(result, Is.EqualTo(0));
+        Assert.That(result, Is.Zero);
     }
 
     /// <summary>
@@ -189,7 +189,7 @@ public void SynchronizeSynchronous_RunsWithAsyncTasksInSubscriptions()
         }
 
         // Then
-        Assert.That(result, Is.EqualTo(0));
+        Assert.That(result, Is.Zero);
     }
 
     /// <summary>
@@ -232,6 +232,6 @@ public void SubscribeAsync_RunsWithAsyncTasksInSubscriptions()
         }
 
         // Then
-        Assert.That(result, Is.EqualTo(0));
+        Assert.That(result, Is.Zero);
     }
 }