MDL-61407 privacy: Reset the writer after writer tests

The writer was not reset after these tests which had the potential to
affect the next test after these were run.

privacy/tests/writer_test.php

@@ -40,6 +40,14 @@
  * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class writer_test extends advanced_testcase {
+    /**
+     * Ensure that the writer is cleared away as appropriate after each
+     * test.
+     */
+    public function tearDown() {
+        writer::reset();
+    }
+
     /**
      * Test that calling with_context multiple times will return the same write instance.
      */