backup/util/helper/tests/cronhelper_test.php changes time zone while performing its tests but does not put time zone back to previous value.
This causes subsequent tests to fail that depend on correct time zone.
Database is Oracle 12.
Moodle version 2.6.6+
(Note: current code in git does not correct this.)
Tests that fail are:
lib/tests/statslib_test.php
enrol/imsenterprise/tests/imsenterprise_test.php

These tests will pass when run individually.