fix: change NEVER_LOCALE to Interlingua (ia)

freedomofpress/securedrop-dev-docs#85 standardizes our use of Esperanto
(eo) as a pseudolocale and Interlingua (ia) as a testing locale.  (Ido
[io] is not available in the CLDR used in our current Babel 2.9.1, so
we'll have to use ia for now.)

securedrop/tests/test_i18n.py

@@ -34,7 +34,9 @@
 from tests.factories import SecureDropConfigFactory
 from werkzeug.datastructures import Headers
 
-NEVER_LOCALE = "eo"  # Esperanto
+# Interlingua, per
+# <https://developers.securedrop.org/en/latest/supported_languages.html>.
+NEVER_LOCALE = "ia"
 
 
 def create_config_for_i18n_test(