#3168858 Replace deprecated drupalPostForm() in tests.

Resolves #3408042 by replacing deprecated drupalPostForm() with drupalGet() and submitForm().

tests/src/Functional/EmptyValuesTest.php

@@ -33,7 +33,8 @@ class EmptyValuesTest extends FileFieldSourcesTestBase {
    */
   public function testAllSourcesEnabled() {
     // Change allowed number of values.
-    $this->drupalPostForm('admin/structure/types/manage/' . $this->typeName . '/fields/node.' . $this->typeName . '.' . $this->fieldName . '/storage', ['cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED], t('Save field settings'));
+    $this->drupalGet('admin/structure/types/manage/' . $this->typeName . '/fields/node.' . $this->typeName . '.' . $this->fieldName . '/storage');
+    $this->submitForm(['cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED], t('Save field settings'));
 
     $this->enableSources([
       'upload' => TRUE,