DX-5153: Add support for custom Twig tests in the Twig validator.

[pfrenssen]

Fixes #4579

Testing steps

1. Add the following code to an existing Twig template:

   <div>{{ myvar is instanceof('\\MyInterface') ? 'yes' : 'no' }}</div>

2. Run the Twig validation. This is expected to throw an error because the instanceof test is not part of core Twig:

   $ blt validate:twig
   ERROR  in /app/docroot/themes/custom/mytheme/templates/mytemplate.html.twig
   >> Unknown "instanceof" test.

3. Now edit blt.yml and add instanceof to the whitelisted Twig tests by adding the following section:

   validate:
     twig:
       tests:
         - instanceof

4. Run the validation again. Now there should not be any errors.

   $ blt validate:twig
   [OK] All 235 Twig files contain valid syntax.

[danepowell]

Can you add the default value for validate.twig.tests to https://github.com/acquia/blt/blob/main/config/build.yml along with a comment explaining how to use it?

[pfrenssen]

Thanks for the quick review and suggestions! I made the requested changes.

[danepowell]

Thanks for the contribution.