Akka.Cluster: added sanity check for Member.AgeComparer (akkadotnet…

…#7291)

we would have seen A LOT of smoke and fire if this didn't work correctly, but since we're in the midst of testing for all sorts of member transition-related issues for akkadotnet#6973 we thought it would be best to add a sanity check.

src/core/Akka.Cluster.Tests/MemberOrderingSpec.cs

@@ -266,6 +266,23 @@ public void LeaderOrdering_must_order_members_with_status_joining_exiting_down_l
             var shuffled = expected.Shuffle().ToImmutableList();
             shuffled.Sort(Member.LeaderStatusOrdering).Should().BeEquivalentTo(expected);
         }
+
+        [Fact]
+        public void MemberAgeOrdering_must_order_members_by_ascending_UpNumber()
+        {
+            var address = new Address("akka.tcp", "sys1", "host1", 5000);
+            var m1 = TestMember.Create(address, MemberStatus.Up, ImmutableHashSet<string>.Empty, upNumber: 1);
+            var m2 = TestMember.Create(address.WithPort(7000), MemberStatus.Up, ImmutableHashSet<string>.Empty, upNumber: 2);
+            var m3 = TestMember.Create(address.WithPort(3000), MemberStatus.Up, ImmutableHashSet<string>.Empty, upNumber: 3);
+            var m4 = TestMember.Create(address.WithPort(6000), MemberStatus.Up, ImmutableHashSet<string>.Empty, upNumber: 4);
+            var m5 = TestMember.Create(address.WithPort(2000), MemberStatus.Up, ImmutableHashSet<string>.Empty, upNumber: 5);
+
+            // shuffle the list so the members aren't initially sorted by order
+            var members = new List<Member>(){ m1, m2, m3, m4, m5 }.Shuffle();
+            var sortedMembers = ImmutableSortedSet<Member>.Empty.WithComparer(Member.AgeOrdering).Union(members);
+            sortedMembers.Should().BeEquivalentTo(new List<Member> { m1, m2, m3, m4, m5 });
+            m1.IsOlderThan(m5).Should().BeTrue();
+        }
     }
 }