Added "/" divider for DATE(TIME)_INPUT_FORMATS for Dutch locale.

Dutch ('nl-NL' and 'nl-BE') has several valid input formats;
see http://nl.wikipedia.org/wiki/Datum_(dagtekening).

Thanks Erik Romijn for review.
Refs #21230 -- removed direct settings manipulation from staticfile tests
Fixed regression from a2dd618e3b4a7472fab852da450ca5eef92a922f.
Fixed #21799 - Modified loaddata --ignorenonexistent to ignore models.
Removed some unnecessary exception naming; thanks claudep.