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/Rulesets/Edit/BeatmapVerifier.cs

@@ -16,8 +16,11 @@ namespace osu.Game.Rulesets.Edit
     {
         private readonly List<ICheck> checks = new List<ICheck>
         {
-            new CheckBackground(),
-            new CheckBackgroundQuality()
+            // Resources
+            new CheckBackgroundPresence(),
+            new CheckBackgroundQuality(),
+            // Audio
+            new CheckAudioPresence(),
         };
 
         public IEnumerable<Issue> Run(WorkingBeatmap workingBeatmap) => checks.SelectMany(check => check.Run(workingBeatmap));