Fix tempnam() usage in PHP7.1 for Drupal 7

Drupal 7 port of #2842942: Fix tempnam() usage in PHP7.1

Problem/Motivation

Using tempnam() in PHP7.1 can result in PHP warnings if it falls back to the system temporary directory. This is the case for some of our tests.

Locale.LocaleExportFunctionalTest
✓		- setUp
✗	
testExportTranslation
exception: [Notice] Line 1343 of modules/locale/locale.test:
tempnam(): file created in the system's temporary directory
✓		- testExportTranslationTemplateFile

See https://bugs.php.net/bug.php?id=69489
See https://www.drupal.org/pift-ci-job/587879

Proposed resolution

Don't use temporary:// as the stream wrapper is not defined.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None