Add test case covering stability of compatibility export operation

This is important as the format will be used more when lazer beatmap
submission comes online, and its stability is a useful property for
that.

Included archive contains an `.osu` with a few fractional-millisecond
timing points and objects, as well as a multi-segment-type slider. That
should cover the range of all possible modifications that the
compatibility exporter currently performs.

osu.Game.Tests/Beatmaps/IO/LegacyBeatmapExporterTest.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System.IO;
+using System.Text;
 using NUnit.Framework;
 using osu.Framework.Allocation;
 using osu.Framework.Extensions;
@@ -9,6 +10,7 @@
 using osu.Framework.Testing;
 using osu.Game.Beatmaps;
 using osu.Game.Database;
+using osu.Game.IO.Archives;
 using osu.Game.Tests.Resources;
 using osu.Game.Tests.Visual;
 using MemoryStream = System.IO.MemoryStream;
@@ -48,6 +50,47 @@ public void TestObjectsSnappedAfterTruncatingExport()
             AddAssert("hit object is snapped", () => beatmap.Beatmap.HitObjects[0].StartTime, () => Is.EqualTo(28519).Within(0.001));
         }
 
+        [Test]
+        public void TestExportStability()
+        {
+            IWorkingBeatmap beatmap = null!;
+            MemoryStream firstExport = null!;
+            MemoryStream secondExport = null!;
+
+            // Ensure importer encoding is correct
+            AddStep("import beatmap", () => beatmap = importBeatmapFromArchives(@"legacy-export-stability-test.olz"));
+            AddStep("export once", () =>
+            {
+                firstExport = new MemoryStream();
+
+                new LegacyBeatmapExporter(LocalStorage)
+                    .ExportToStream((BeatmapSetInfo)beatmap.BeatmapInfo.BeatmapSet!, firstExport, null);
+            });
+
+            AddStep("import beatmap again", () => beatmap = importBeatmapFromStream(firstExport));
+            AddStep("export again", () =>
+            {
+                secondExport = new MemoryStream();
+
+                new LegacyBeatmapExporter(LocalStorage)
+                    .ExportToStream((BeatmapSetInfo)beatmap.BeatmapInfo.BeatmapSet!, secondExport, null);
+            });
+
+            const string osu_filename = @"legacy export - stability test (spaceman_atlas) [].osu";
+
+            AddAssert("exports are identical",
+                () => getStringContentsOf(osu_filename, firstExport.GetBuffer()),
+                () => Is.EqualTo(getStringContentsOf(osu_filename, secondExport.GetBuffer())));
+
+            string getStringContentsOf(string filename, byte[] archiveBytes)
+            {
+                using var memoryStream = new MemoryStream(archiveBytes);
+                using var archiveReader = new ZipArchiveReader(memoryStream);
+                byte[] fileContent = archiveReader.GetStream(filename).ReadAllBytesToArray();
+                return Encoding.UTF8.GetString(fileContent);
+            }
+        }
+
         private IWorkingBeatmap importBeatmapFromStream(Stream stream)
         {
             var imported = beatmaps.Import(new ImportTask(stream, "filename.osz")).GetResultSafely();