tests: Add an en_US locale to the test runtime

tests/make-test-runtime.sh

@@ -26,5 +26,11 @@ for i in `sort -u $T`; do
     cp "$i" ${DIR}/usr/lib/
 done
 
+# We copy the C.UTF8 locale and call it en_US. Its a bit of a lie, but
+# the real en_US locale is often not available, because its in the
+# local archive.
+mkdir -p ${DIR}/usr/lib/locale/
+cp -r /usr/lib/locale/C.* ${DIR}/usr/lib/locale/en_US
+
 flatpak build-export --runtime repo ${DIR}
 rm -rf ${DIR}