Add some model tests (#9)

Reviewed-on: #9

test/fixtures/expenses.yml

@@ -1,15 +1,29 @@
 # Read about fixtures at https://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
 
-one:
-  description: MyString
-  payment: 9.99
-  period: 1
+monthly_expense:
+  description: Monthly Expense
+  payment: 120.00
+  period: monthly
+  credit_card: true
+  estimated: false
+
+annual_expense:
+  description: Annual Expense
+  payment: 120.00
+  period: annually
+  credit_card: true
+  estimated: false
+
+weekly_expense:
+  description: Weekly Expense
+  payment: 120.00
+  period: weekly
   credit_card: false
   estimated: false
 
-two:
-  description: MyString
-  payment: 9.99
-  period: 1
+quarterly_expense:
+  description: Quarterly Expense
+  payment: 120.00
+  period: quarterly
   credit_card: false
   estimated: false