Factor out general file presence checking

This allows us to use the same method of checking for other files that should exist, for example the audio file.

By using the same method, they all share test cases too.

osu.Game.Tests/Editing/Checks/CheckFilePresenceTest.cs

@@ -0,0 +1,68 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Generic;
+using System.Linq;
+using NUnit.Framework;
+using osu.Game.Beatmaps;
+using osu.Game.Rulesets.Edit.Checks;
+using osu.Game.Rulesets.Objects;
+using osu.Game.Tests.Beatmaps;
+
+namespace osu.Game.Tests.Editing.Checks
+{
+    [TestFixture]
+    public class CheckFilePresenceTest
+    {
+        private CheckBackgroundPresence check;
+        private WorkingBeatmap beatmap;
+
+        [SetUp]
+        public void Setup()
+        {
+            check = new CheckBackgroundPresence();
+            beatmap = new TestWorkingBeatmap(new Beatmap<HitObject>
+            {
+                BeatmapInfo = new BeatmapInfo
+                {
+                    Metadata = new BeatmapMetadata { BackgroundFile = "abc123.jpg" },
+                    BeatmapSet = new BeatmapSetInfo
+                    {
+                        Files = new List<BeatmapSetFileInfo>(new[]
+                        {
+                            new BeatmapSetFileInfo { Filename = "abc123.jpg" }
+                        })
+                    }
+                }
+            });
+        }
+
+        [Test]
+        public void TestBackgroundSetAndInFiles()
+        {
+            Assert.That(check.Run(beatmap), Is.Empty);
+        }
+
+        [Test]
+        public void TestBackgroundSetAndNotInFiles()
+        {
+            beatmap.BeatmapInfo.BeatmapSet.Files.Clear();
+
+            var issues = check.Run(beatmap).ToList();
+
+            Assert.That(issues, Has.Count.EqualTo(1));
+            Assert.That(issues.Single().Template is CheckFilePresence.IssueTemplateDoesNotExist);
+        }
+
+        [Test]
+        public void TestBackgroundNotSet()
+        {
+            beatmap.Metadata.BackgroundFile = null;
+
+            var issues = check.Run(beatmap).ToList();
+
+            Assert.That(issues, Has.Count.EqualTo(1));
+            Assert.That(issues.Single().Template is CheckFilePresence.IssueTemplateNoneSet);
+        }
+    }
+}