Fix template tags missing when the tags module is imported during test collection

[izquierdo]

I'm hitting this error

django.template.exceptions.TemplateSyntaxError: 'l10n' is not a registered tag library.

when trying to load Django's l10n library in a template while using pytest and django_coverage_plugin. Tests pass when the plugin is not used. This started happening after adding from django.templatetags.l10n import localize to a test module.

This happens in Python 3.6 with Django 1.11.

From my investigation of the issue, it's triggered in the following way:

• pytest's test collector imports the test module
• The test module imports the l10n template tag
• The tracer kicks in because templatetags modules are included since 'template' is a prefix of 'templatetags' and this causes check_debug() to be run at a time when django.template.engines hasn't imported the engines yet.
• This line triggers loading of the engines through https://github.com/django/django/blob/1.11.21/django/template/utils.py#L80
• Because we're still in the middle of importing l10n, this import works but the module does not yet have a register attribute
• Thus Django considers l10n not to be a template tags module and does not register it and considers the template engine to be fully loaded.

I believe the root issue here is that check_debug() is trying to import template engines too early and that the if not apps.ready: check is not enough. Unfortunately I couldn't find any other way to detect that the engines have been loaded other than by looking at the internal variable _engines.

Can confirm that this fixes #63 using the test case attached there.

Wasn't able to add a test to this PR as the bug depends on pytest's collector.

[nedbat]

Thanks, I've cherry-picked the fix commit as 6622791.