Partially refactor Language switcher block tests

Problem/Motivation

This is a follow-up issue to 🐛 [regression] Language switcher block throws exception when no route is matched Fixed , where the tests got added.

The issue-fork added a couple of tests which need to get refactored.

From that issue (note that the comment is aimed at "core/modules/language/tests/src/Functional/LanguageSwitchingTest.php"):

Note, that at the start of "testRestrictedPaths()" we are still logged in as the created "admin_user", the test only succeeds, because this user is not a "real" admin user, since he only has the following permissions:

      'administer blocks',
      'administer languages',
      'access administration pages',
      'access content',

This makes the test quite hard to understand, since later in this test we log in as a "privileged user" even if the admin should be already privileged with the required permissions.

Furthermore, I do not understand the assertion in line 560:

    // Visit a restricted user page.
    // Assert that the language switching block is displayed on the
    // access-denied page, but it does not contain the path alias.
    $this->assertLinkMarkup('/user/1', 403, $block->label(), 'peter-parker');

Why would a block be displayed on a 403 page? Am I missing something here?

Steps to reproduce

Proposed resolution

Fix the mentioned issues

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet