EZP-21435/21436: ezpDatabaseTestCase: Use global eZDB instance, if available

The tests for the ezcomments and ezfind extensions fail because `$this->sharedFixture` is not set.

eZ Comments: https://travis-ci.org/jeromegamez/ezpublish-legacy-tests/jobs/10397894#L331
eZ Find: https://travis-ci.org/jeromegamez/ezpublish-legacy-tests/jobs/10747489#L419

When running the eZ Publish tests, the shared fixture has already been set in `ezpDatabaseTestSuite`, so the sharedFixture can be retrieved from there.

Cheers
:octocat: Jérôme