Revert "Remove migration testing"

This reverts commit 5eb691fd.

ci/migrate-testing

+#!/bin/sh
+# Migration ordering testing
+set -e
+set -x
+
+for app in trans lang accounts ; do 
+    rm -f weblate.db
+    ./manage.py migrate --noinput --settings weblate.settings_test sites
+    ./manage.py migrate --settings weblate.settings_test --traceback $app
+done

ci/run-test

@@ -4,5 +4,8 @@ set -e
 set -x
 
 ./manage.py check
+if [ "$CI_DATABASE" = sqlite -o -z "$CI_DATABASE" ] ; then 
+    ./ci/migrate-testing
+fi
 ./manage.py migrate --noinput --traceback
 coverage run --source=. ./manage.py test -v 2 $CI_RUN_TESTS