CRM_Core_Menu - Fix warnings during local test run (dev/core#742)

CRM_Core_Menu - Fix warnings during local test run (dev/core#742)

Before
------

In certain local OSX+D8 test environments, when running the full suite, I got a large number of warnings like this:

```
Warning: simplexml_load_file(): I/O warning : failed to load external entity "/Users/totten/bknix/build/drupal8-clean/web/vendor/civicrm/civicrm-core//CRM/Campaign/xml/Menu/Campaign.xml" in /Users/totten/bknix/build/drupal8-clean/web/vendor/civicrm/civicrm-core/CRM/Core/Menu.php on line 99
    ```

After
-----

Those warnings seem to have gone away.

The menu still seems to populate correctly.

Comments
--------

This smells a lot like dev/core#742 -- e.g.  it's the same warning and
(after adjusting for changes in line-numbering over the months) same line of
code.