R1:29cbb0a6580f

Make i18n string extraction ignore strings in test case files

Summary:
Do not extract strings from test case files for localization / translation.
They are not exposed to users, and localizing wastes translators' time.

Closes T15815

Test Plan:
* Install Wikimedia downstream `translations` extension via `git clone https://gerrit.wikimedia.org/r/phabricator/translations` and put it on the same filesystem level as `phorge` and `arcanist`, and follow https://we.phorge.it/w/docs/extensions/install/
* Apply…