Fix FileBackendGroup test for some configurations

Fix FileBackendGroup test for some configurations

The expected value of FileBackendGroup::config()['srvCache'] was being
obtained from MediaWikiServices::getLocalServerObjectCache(), but the
class actually used ObjectCache::getLocalServerInstance( 'hash' ). It
happens these are often the same, so it passed the gate-and-submit jobs,
but it's causing failures on Travis CI.

Follow-up to bd2a4395025, which added the failing test. Thanks to
Krinkle for pointing out the failures on CI.

Change-Id: I17651766f4ee2752dfcae9574d2538269dec4ebe