Let Options open at any max instances that can be set

src/DiffEngineTray.Tests/OptionsFormMaxInstancesTests.cs

@@ -0,0 +1,27 @@
+using NumericUpDown = System.Windows.Forms.NumericUpDown;
+
+/// <summary>
+/// The form's range has to be the setting's range. DiffEngine_MaxInstances takes any ushort, and
+/// the spinner was left at NumericUpDown's default maximum of 100, so anything above that threw
+/// out of the constructor and Options could never be opened to lower it again.
+/// </summary>
+[TUnit.Core.Executors.STAThreadExecutor]
+public class OptionsFormMaxInstancesTests
+{
+    [Test]
+    public async Task Opens_at_the_highest_max_instances_that_can_be_set()
+    {
+        using var form = new OptionsForm(
+            new()
+            {
+                MaxInstancesToLaunch = ushort.MaxValue
+            },
+            _ => Task.FromResult<IReadOnlyCollection<string>>([]));
+
+        var spinner = form.Controls
+            .Find("maxInstancesNumericUpDown", true)
+            .OfType<NumericUpDown>()
+            .Single();
+        await Assert.That(spinner.Value).IsEqualTo(ushort.MaxValue);
+    }
+}