Clear up cached Main_page title in test

In Echo tests/NotificationFormatterTest.php there is a test that ends up
invoking the Linker to create full URL for Main_page and pass it a
fragment.

Title::newMainPage() is really Title::newFromText('Main_Page') which is
cached.  Whenever the fragment is set, it stuck in the cache entry and
later call to Title::newMainPage() yields a Title object which has the
fragment.

That cause Scribunto tests (at least) to fail when Echo tests are run
before it because Scribunto tests expect a Main_Page title without a
fragment.

The ugly workaround is to have the Echo test to reset the fragment.

We could clear up the Title cache between tests though.

Bug: 68646
Change-Id: I625492fc8bf0dbd6e10275c3a728f5c1f85768c8
Merge "Clear up cached Main_page title in test"