Add browser test for preview functionality to MediaWiki core

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