Fix tests on 2.0.x branch

Also:
https://git.drupalcode.org/project/metatag/-/jobs/2134642

The two test failures:

ReflectionException: Class "Drupal\Tests\metatag\Unit\MetaNameBaseTest" does not exist in /builds/project/metatag/web/core/scripts/run-tests.sh:820
Stack trace:
#0 /builds/project/metatag/web/core/scripts/run-tests.sh(820): ReflectionClass->__construct('Drupal\\Tests\\me...')
#1 /builds/project/metatag/web/core/scripts/run-tests.sh(846): simpletest_script_run_phpunit(Object(Drupal\Core\Test\TestRun), 'Drupal\\Tests\\me...')
#2 /builds/project/metatag/web/core/scripts/run-tests.sh(68): simpletest_script_run_one_test(Object(Drupal\Core\Test\TestRun), 'Drupal\\Tests\\me...')
#3 {main}FATAL Drupal\Tests\metatag\Unit\MetaNameBaseTest: test runner returned a non-zero error code (2).
Drupal\Tests\metatag\Unit\MetaNameBaseTest                     0 passes   1 fails                            
Drupal\Tests\metatag_mobile\Functional\TagsTest                1 passes                                      
ReflectionException: Class "Drupal\Tests\metatag\Unit\MetatagTrimmerTest" does not exist in /builds/project/metatag/web/core/scripts/run-tests.sh:820
Stack trace:
#0 /builds/project/metatag/web/core/scripts/run-tests.sh(820): ReflectionClass->__construct('Drupal\\Tests\\me...')
#1 /builds/project/metatag/web/core/scripts/run-tests.sh(846): simpletest_script_run_phpunit(Object(Drupal\Core\Test\TestRun), 'Drupal\\Tests\\me...')
#2 /builds/project/metatag/web/core/scripts/run-tests.sh(68): simpletest_script_run_one_test(Object(Drupal\Core\Test\TestRun), 'Drupal\\Tests\\me...')
#3 {main}FATAL Drupal\Tests\metatag\Unit\MetatagTrimmerTest: test runner returned a non-zero error code (2).
Drupal\Tests\metatag\Unit\MetatagTrimmerTest                   0 passes   1 fails                            

I did fix the class namespace issues, but now it's failing randomly, e.g.:

https://git.drupalcode.org/project/metatag/-/jobs/2135061
- Fails on MetatagViewsTokenTest because "Title found".

https://git.drupalcode.org/project/metatag/-/jobs/2135015
- Fails on MaintenanceModeTest because "The string "Access denied | " appears in the HTML response of this page, but it should not"

Argh.

It finally executed correctly: https://git.drupalcode.org/project/metatag/-/jobs/2135241

And it finished in under three minutes!