MDL-60923 backup: Added $CFG->backuptempdir

The new setting will allow to host the temporary backup files
into a specific target directory. Defaults to '$CFG->tempdir/backup'.

Calling make_backup_temp_directory() checks that the required sub-directory
will be properly created under the new target directory.

backup/controller/tests/controller_test.php

@@ -110,7 +110,7 @@ class core_backup_controller_testcase extends advanced_testcase {
         global $CFG;
 
         // Make a backup.
-        check_dir_exists($CFG->tempdir . '/backup');
+        make_backup_temp_directory('');
         $bc = new backup_controller(backup::TYPE_1ACTIVITY, $this->moduleid, backup::FORMAT_MOODLE,
             backup::INTERACTIVE_NO, backup::MODE_IMPORT, $this->userid);
         $backupid = $bc->get_backupid();
@@ -159,7 +159,7 @@ class core_backup_controller_testcase extends advanced_testcase {
 
         $foldername = 'deadlock';
         $fp = get_file_packer('application/vnd.moodle.backup');
-        $tempdir = $CFG->dataroot . '/temp/backup/' . $foldername;
+        $tempdir = make_backup_temp_directory($foldername);
         $files = $fp->extract_to_pathname($CFG->dirroot . '/backup/controller/tests/fixtures/deadlock.mbz', $tempdir);
 
         $this->setAdminUser();