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