Add Timelines serialization test

MultiversalDiplomacyTests/SerializationTest.cs

@@ -14,6 +14,31 @@ public class SerializationTest
         WriteIndented = true,
     };
 
+    [Test]
+    public void SerializeRoundTrip_Timelines()
+    {
+        Timelines one = Timelines.Create();
+
+        string serial1 = JsonSerializer.Serialize(one, Options);
+        Timelines two = JsonSerializer.Deserialize<Timelines>(serial1, Options)
+            ?? throw new AssertionException("Failed to deserialize");
+
+        Assert.That(two.Next, Is.EqualTo(one.Next), "Failed to reserialize next timeline");
+        Assert.That(two.Pasts, Is.EquivalentTo(one.Pasts), "Failed to reserialize pasts");
+
+        Timelines three = two
+            .WithNewSeason(Season.First, out var a1)
+            .WithNewSeason(a1, out var a2)
+            .WithNewSeason(a1, out var b2);
+
+        string serial2 = JsonSerializer.Serialize(three, Options);
+        Timelines four = JsonSerializer.Deserialize<Timelines>(serial2, Options)
+            ?? throw new AssertionException("Failed to deserialize");
+
+        Assert.That(four.Next, Is.EqualTo(three.Next), "Failed to reserialize next timeline");
+        Assert.That(four.Pasts, Is.EquivalentTo(three.Pasts), "Failed to reserialize pasts");
+    }
+
     [Test]
     public void SerializeRoundTrip_NewGame()
     {