Improve flaky SiteStatsTest

Improve flaky SiteStatsTest

SiteStatsTest::testJobsCountGetCached() is somewhat flaky in that if it
runs after a test that adds a page (thereby producing htmlCacheUpdate
and recentChangesUpdate jobs) but doesn't have the CI framework reset
the `page` tables (which has the side effect of clearing all such jobs),
it will fail.

This change manually clears those jobs so it doesn't depend on test
ordering.

Change-Id: I1277e633c81b29bda7564fa12d23f13ded7298c7