Fix broken filter function and failing test

Problem/Motivation

Update: Manual testing shows that the filter function doesn't seem to work as expected, so the test seems to be right!

One test is still failing, where we'd expect the trademark to be filtered out:

Testing Drupal\Tests\drowl_trademark\FunctionalJavascript\DrowlTrademarkFunctionalJavascriptTest
..F                                                                 3 / 3 (100%)

Time: 00:18.039, Memory: 4.00 MB

There was 1 failure:

1) Drupal\Tests\drowl_trademark\FunctionalJavascript\DrowlTrademarkFunctionalJavascriptTest::testTrademarkInExcludedClass
Failed asserting that the text of the element identified by 'article > div > div > p' equals 'prefix test suffix'.
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'prefix test suffix'
+'prefix test® suffix'

/var/www/html/vendor/phpunit/phpunit/src/Framework/Constraint/Equality/IsEqual.php:96
/var/www/html/core/tests/Drupal/Tests/WebAssert.php:995
/var/www/html/modules/contrib/drowl_trademark/tests/src/FunctionalJavascript/DrowlTrademarkFunctionalJavascriptTest.php:140
/var/www/html/vendor/phpunit/phpunit/src/Framework/TestResult.php:726

FAILURES!
Tests: 3, Assertions: 27, Failures: 1.

https://www.drupal.org/pift-ci-job/2344609

We should test manually that it works as expected and then try to find out, why it doesn't work in the test or what's the reason. Testing in the test would need to much time until we know the deeper reason.

Steps to reproduce

Test the filter functionality

Proposed resolution

Check and fix the filter functionality

Remaining tasks

Debug and fix the filter functionality
Write further tests for other nesting structures to ensure it works as expected

User interface changes

API changes

Data model changes