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 https://gerrit.wikimedia.org/r/c/phabricator/translations/+/1028887 on top, to make the translations extension work with upstream Phorge.
• In the downstream translations extension path, run ./export.sh, then run grep -r "__tests__" . and get a list of qqq.json files with context for strings from test files.
• Apply this patch.
• In the downstream translations extension path, run ./export.sh, then run grep -r "__tests__" . to verify that no qqq.json files include any strings from test files anymore.

Reviewers: O1 Blessed Committers, aklapper, avivey

Reviewed By: O1 Blessed Committers, aklapper, avivey

Subscribers: avivey, aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15815

Differential Revision: https://we.phorge.it/D25624