Intermittent test failures

Seen both locally and on Travis:

```
3 of 581 tests failed:
  1) Content Delete content verify file delete:
     TypeError: Cannot read property 'alias' of undefined
      at _produceContentSearchDocument (/home/travis/build/sakaiproject/Hilary/node_modules/oae-content/lib/search.js:245:38)
      at _produceContentSearchDocuments (/home/travis/build/sakaiproject/Hilary/node_modules/oae-content/lib/search.js:144:27)
      at Array.forEach (native)
      at Function._.each._.forEach (/home/travis/build/sakaiproject/Hilary/node_modules/underscore/underscore.js:78:11)
      at _produceContentSearchDocuments (/home/travis/build/sakaiproject/Hilary/node_modules/oae-content/lib/search.js:143:15)
      at _getRevisionItems (/home/travis/build/sakaiproject/Hilary/node_modules/oae-content/lib/search.js:169:16)
      at _produceContentSearchDocuments (/home/travis/build/sakaiproject/Hilary/node_modules/oae-content/lib/search.js:138:9)
      at _getContentItems (/home/travis/build/sakaiproject/Hilary/node_modules/oae-content/lib/search.js:221:9)
      at module.exports.getMultipleContentItems (/home/travis/build/sakaiproject/Hilary/node_modules/oae-content/lib/internal/dao.content.js:78:13)
      at executeQuery (/home/travis/build/sakaiproject/Hilary/node_modules/oae-util/lib/cassandra.js:725:9)
  2) File previews verify negative preview expires are not possible:
     Error: timeout of 40000ms exceeded
      at Object.<anonymous> (/home/travis/build/sakaiproject/Hilary/node_modules/mocha/lib/runnable.js:167:14)
      at Timer.list.ontimeout (timers.js:101:19)
  3) Related content "before all" hook:
     Error: timeout of 40000ms exceeded
      at Object.<anonymous> (/home/travis/build/sakaiproject/Hilary/node_modules/mocha/lib/runnable.js:167:14)
      at Timer.list.ontimeout (timers.js:101:19)
```

When the first test succeeds, the other two never fail.