Don't run all template tests with string_if_invalid='INVALID'

[Tim Graham]

After the template tests refactor in #23768, I don't think there's a need to run all the template tests with a different value of string_if_invalid (see template_tests/syntax_tests/utils.py. Instead of if self.engine.string_if_invalid: in a given test, we can add a second assertion to these tests and use self.settings(TEMPLATES={... string_if_invalid='INVALID' ...) and then render the template a second time.

[Aymeric Augustin]

I refactored the template engine into a library in order to be able to test it without depending on Django settings.

I'm now working to refactor the new individual tests to stop using Django settings.

Can you hold on for a few weeks?

Once I've merged my multiple template engines branch, we can re-evaluate refactoring opportunities in this area. Thanks.

[Tim Graham]

I updated the description to account for the TEMPLATE_STRING_IF_INVALID setting moving into TEMPLATES OPTIONS. The issue ​is still relevant although the fix may not look so nice since we'd have to override the TEMPLATES setting (list of dictionaries) instead of boolean setting.