add unit test for assumption about HashSet<T> order

This will let us detect if the behavior ever changes in a future version of .NET, so we need to change the new ModsToLoadEarly/Late config fields.

src/SMAPI.Tests/Core/AssumptionTests.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using FluentAssertions;
+using FluentAssertions.Execution;
+using NUnit.Framework;
+using StardewModdingAPI.Framework.Models;
+
+namespace SMAPI.Tests.Core
+{
+    /// <summary>Unit tests which validate assumptions about .NET used in the SMAPI implementation.</summary>
+    [TestFixture]
+    internal class AssumptionTests
+    {
+        /*********
+        ** Unit tests
+        *********/
+        /****
+        ** Constructor
+        ****/
+        [Test(Description = $"Assert that {nameof(HashSet<string>)} maintains insertion order when no elements are removed. If this fails, we'll need to change the implementation for the {nameof(SConfig.ModsToLoadEarly)} and {nameof(SConfig.ModsToLoadLate)} options.")]
+        [TestCase("construct from array")]
+        [TestCase("add incrementally")]
+        public void HashSet_MaintainsInsertionOrderWhenNoElementsAreRemoved(string populateMethod)
+        {
+            // arrange
+            string[] inserted = Enumerable.Range(0, 1000)
+                .Select(_ => Guid.NewGuid().ToString("N"))
+                .ToArray();
+
+            // act
+            HashSet<string> set;
+            switch (populateMethod)
+            {
+                case "construct from array":
+                    set = new(inserted, StringComparer.OrdinalIgnoreCase);
+                    break;
+
+                case "add incrementally":
+                    set = new(StringComparer.OrdinalIgnoreCase);
+                    foreach (string value in inserted)
+                        set.Add(value);
+                    break;
+
+                default:
+                    throw new AssertionFailedException($"Unknown populate method '{populateMethod}'.");
+            }
+
+            // assert
+            string[] actualOrder = set.ToArray();
+            actualOrder.Should().HaveCount(inserted.Length);
+            for (int i = 0; i < inserted.Length; i++)
+            {
+                string expected = inserted[i];
+                string actual = actualOrder[i];
+
+                if (actual != expected)
+                    throw new AssertionFailedException($"The hash set differed at index {i}: expected {expected}, but found {actual} instead.");
+            }
+        }
+    }
+}