Build form properly for FinancialItemTest::testGetPreviousFinancialItemHavingTax

tests/phpunit/CRM/Financial/BAO/FinancialItemTest.php

@@ -268,6 +268,7 @@ class CRM_Financial_BAO_FinancialItemTest extends CiviUnitTestCase {
       'contribution_status_id' => 1,
       'price_set_id' => 0,
     ]);
+    $form->buildForm();
     $form->postProcess();
     $contribution = $this->callAPISuccessGetSingle('Contribution',
       [