add more tests to models

app/models/mention.rb

@@ -1,11 +1,10 @@
 # frozen_string_literal: true
 
 class Mention < ApplicationRecord
-  belongs_to :account, inverse_of: :mentions
-  belongs_to :status
+  belongs_to :account, inverse_of: :mentions, required: true
+  belongs_to :status, required: true
 
   has_one :notification, as: :activity, dependent: :destroy
 
-  validates :account, :status, presence: true
   validates :account, uniqueness: { scope: :status }
 end