Adjust GlobalBlocking database under Wikimedia Jenkins

GlobalBlocking is meant to use a dedicated shared database
'globalblocking'. When run under Jenkins harness, we only have one
database, whenever another tests hits GlobalBlocking (such as Cite
parser tests), we end up with stracktrace because the database load
balancer can not find 'globalblocking' database.

Luckily:
- the GlobalBlocking database can be configured via
  $wgGlobalBlockingDatabase
- Wikimedia Jenkins set $wgWikimediaJenkinsCI

So we can just point $wgGlobalBlockingDatabase to $wgDBname whenever
$wgWikimediaJenkinsCI is set.

Test case:

    $ ls extensions
    Cite
    GlobalBlocking
    $ cd tests/phpunit
    $ php phpunit.php --testsuite extensions

Bug: 67353
Change-Id: I1f9e9657f0c784418a356f18a02b73ecb4ba8c48