PHPUNIT: Make Functional tests more robust

Problem/Motivation

See current code:
https://git.drupalcode.org/project/block_class/-/blob/3.0.x/tests/src/Fu...

    // Assert the custom class in the content block.
    if (version_compare(\Drupal::VERSION, '10', '<')) {
      // Support tests for D9.
      $assert->responseContains('<div id="block-mainpagecontent" class="' . $test_classes_block_main . ' block block-system block-system-main-block">');
      // Assert the custom class in user menu.
      $assert->responseContains('<nav  id="block-useraccountmenu" class="' . $test_classes_block_user_menu . ' block block-menu navigation menu--account secondary-nav" aria-labelledby="block-useraccountmenu-menu" role="navigation">');
    }
    else {
      $assert->responseContains('<div id="block-' . $this->defaultTheme . '-mainpagecontent" class="' . $test_classes_block_main . ' block block-system block-system-main-block">');
      // Assert the custom class in user menu.
      $assert->responseContains('<nav  id="block-' . $this->defaultTheme . '-useraccountmenu" class="' . $test_classes_block_user_menu . ' block block-menu navigation menu--account secondary-nav" aria-labelledby="block-' . $this->defaultTheme . '-useraccountmenu-menu" role="navigation">');
    }

Currently Functional tests are checking for the exact markup in the response, with all the classes, attributes, etc...

The problem is that attributes can change position depending on the Drupal core versions, themes and there is no interest in checking any attributes other than CSS classes.

Steps to reproduce

Run module's PHPUNIT Functional Tests.

Proposed resolution

Replace the use of responseContains with something a bit more flexible, such as:
elementExists('xpath')
to check the existence of the tested HTML element, identified by its id and class attributes.