Fix broken test

Problem/Motivation

There was 1 error:
1) Drupal\Tests\drowl_layouts_bs\Functional\DrowlLayoutsFunctionalTest::testUsingLayout
Exception: Warning: Undefined property: Drupal\Core\Extension\Extension::$base_theme
drowl_layouts_bs_library_info_alter()() (Line: 50)

Looks like this commit broke it:
https://git.drupalcode.org/project/drowl_layouts_bs/-/commit/95e2f893a3e...

Maybe the ->base_theme was a bit too optimistic?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes