(bug 37209) Make TextContentTest and WikitextContentTest work in all cases

(bug 37209) Make TextContentTest and WikitextContentTest work in all cases

Currently TextContentTest::testDeletionUpdates() and
WikitextContentTest::testGetSecondaryDataUpdates() set a random page ID to
pages that does not exist. This works in most cases, but when a method like
Title::getLatestRevID() is called on these objects, it throws an excpection
"LinkCache doesn't currently knows about this title."
This happens e.g. when SemanticMediaWiki extension is installed.

Intsead of setting a random page ID, really insert the page in the database
before getting table updates so that it won't crash anymore.

Change-Id: I489c406f78897bc38ac41d8d599b778b47b30021