refactor(test): use test fixture in purchase invoice

erpnext/accounts/doctype/purchase_invoice/test_purchase_invoice.py

@@ -5,7 +5,7 @@
 import unittest
 
 import frappe
-from frappe.tests.utils import change_settings
+from frappe.tests.utils import FrappeTestCase, change_settings
 from frappe.utils import add_days, cint, flt, getdate, nowdate, today
 
 import erpnext
@@ -38,7 +38,7 @@
 test_ignore = ["Serial No"]
 
 
-class TestPurchaseInvoice(unittest.TestCase, StockTestMixin):
+class TestPurchaseInvoice(FrappeTestCase, StockTestMixin):
 	@classmethod
 	def setUpClass(self):
 		unlink_payment_on_cancel_of_invoice()
@@ -48,6 +48,9 @@ def setUpClass(self):
 	def tearDownClass(self):
 		unlink_payment_on_cancel_of_invoice(0)
 
+	def tearDown(self):
+		frappe.db.rollback()
+
 	def test_purchase_invoice_received_qty(self):
 		"""
 		1. Test if received qty is validated against accepted + rejected