filemetadatawidgettest: Enforce en_US locale

Test Plan:
`$ make test`
`$ LC_ALL=C make test`
`$ LC_ALL=de_DE.UTF-8 make test`

Reviewers: elvisangelaccio, #baloo, #frameworks

Reviewed By: elvisangelaccio

Subscribers: broulik

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D11238