Adding unit tests

Services/UserService.cs

@@ -17,21 +17,21 @@ public class UserService
         _users = database.GetCollection<User>("Users");
     }
 
-    public async Task<User> GetByUsernameAsync(string username) =>
+    public virtual async Task<User> GetByUsernameAsync(string username) =>
         await _users.Find(u => u.Username == username).FirstOrDefaultAsync();
 
-    public async Task<List<User>> GetAllAsync() =>
+    public virtual async Task<List<User>> GetAllAsync() =>
         await _users.Find(_ => true).ToListAsync();
 
-    public async Task<bool> DeleteAsync(string username)
+    public virtual async Task<bool> DeleteAsync(string username)
     {
         var result = await _users.DeleteOneAsync(u => u.Username == username);
         return result.DeletedCount > 0;
     }
 
-    public async Task CreateAsync(User user) =>
+    public virtual async Task CreateAsync(User user) =>
         await _users.InsertOneAsync(user);
 
-    public async Task UpdateAsync(User user) =>
+    public virtual async Task UpdateAsync(User user) =>
         await _users.ReplaceOneAsync(u => u.Id == user.Id, user);
 }