Add / allow injecting experimental mode param in LanguageFallbackChainFactory

also adds simple test. more/better tests could be done as follow-up.

and fixes codesniffer issue "Must use "self::" for local static member reference"

https://scrutinizer-ci.com/g/wikimedia/mediawiki-extensions-Wikibase/inspections/4cc150eb-7315-4b95-b2cf-f2687f6ac816/issues/

Change-Id: Id20d4512254249aaccae54f0de6c76ab5139e708
Merge "Add / allow injecting experimental mode param in LanguageFallbackChainFactory"