MDL-46375 core_files: Correct filename in mbz test

I noticed during the file system abstraction that this test was
incorrect.

Since both $storagefalse, and $storagetrue are in the same context,
component, area, itemid, and folder, the fact that they had the same
filename meant that they constantly overwrote one another.

As part of archive_to_storage, existing files in the same location are
found, the files themselves deleted, and the existing file record in the
files table is deleted.

The tests continued to pass because:
* the existing variables were not affected by the deletion of the file
  record and file so the comparisons were successful; and
* subsequent calls to fetch the content of the file meant that the
  files themselves were restored from the trash directory.

lib/filestorage/tests/mbz_packer_test.php

@@ -54,7 +54,7 @@ public function test_archive_with_both_options() {
         $this->assertNotEmpty($packer->archive_to_pathname($files, $filetrue));
         $context = context_system::instance();
         $this->assertNotEmpty($storagetrue = $packer->archive_to_storage(
-                $files, $context->id, 'phpunit', 'data', 0, '/', 'false.mbz'));
+                $files, $context->id, 'phpunit', 'data', 0, '/', 'true.mbz'));
 
         // Check the sizes are different (indicating different formats).
         $this->assertNotEquals(filesize($filefalse), filesize($filetrue));