Add browser test for preview functionality to MediaWiki core
The basic functionality of being able to preview an edit is currently
not covered by a test, as far as I can see.
The assertion for a wpTextbox2 that should *not* be there is a result of
the issue documented at T209012, where the EditPage::isConflict flag was
accidentially set. This assertion makes sure accidential conflicts can't
happen again, no matter which extension might cause it.
Bug: T210758
Change-Id: Iae723430b3a88079ad3499429e65c29817eca67e