Fix SQLite failing to initialize on test projects

osu.Game/Database/OsuDbContext.cs

@@ -33,6 +33,12 @@ namespace osu.Game.Database
 
         private static readonly Lazy<OsuDbLoggerFactory> logger = new Lazy<OsuDbLoggerFactory>(() => new OsuDbLoggerFactory());
 
+        static OsuDbContext()
+        {
+            // required to initialise native SQLite libraries on some platforms.
+            SQLitePCL.Batteries_V2.Init();
+        }
+
         /// <summary>
         /// Create a new in-memory OsuDbContext instance.
         /// </summary>