test: uses accessor instead of private property
MediaWikiTestCase::$useTemporaryTables is private variable to prevent
childs from overriding the value. That in turns cause the dump test
suite to fail :/
MediaWiki core provides an accessor with:
https://gerrit.wikimedia.org/r/54468
bug: 46285
Change-Id: I45826631ab1292a16f66a17beff46f924e26c568