Address Deprecated Code in Tests

Problem/Motivation

There are a number of deprecations I'm seeing when running tests:

$ ../vendor/phpunit/phpunit/phpunit -c core/ modules/contrib/codemirror_editor/ --verbose
PHPUnit 9.5.5 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.3.25-1+0~20201130.73+debian10~1.gbp042074
Configuration: /var/www/html/web/core/phpunit.xml.dist
Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

Testing /var/www/html/web/modules/contrib/codemirror_editor
..............                                                    14 / 14 (100%)

Time: 04:34.194, Memory: 8.00 MB

OK (14 tests, 177 assertions)

Remaining self deprecation notices (29)

  6x: The Drupal\Tests\codemirror_editor\Kernel\ModeManagerTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    6x in DrupalListener::startTest from Drupal\Tests\Listeners

  6x: Declaring ::setUp without a void return typehint in Drupal\Tests\codemirror_editor\Kernel\ModeManagerTest is deprecated in drupal:9.0.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3114724
    6x in DrupalListener::startTest from Drupal\Tests\Listeners

  3x: Calling Drupal\Tests\UiHelperTrait::drupalPostForm() with $path set to NULL is deprecated in drupal:9.2.0 and the method is removed in drupal:10.0.0. Use $this->submitForm() instead. See https://www.drupal.org/node/3168858
    2x in TextEditorTest::testTextEditor from Drupal\Tests\codemirror_editor\FunctionalJavascript
    1x in FilterTest::testFilter from Drupal\Tests\codemirror_editor\FunctionalJavascript

  1x: The Drupal\Tests\codemirror_editor\FunctionalJavascript\WidgetFormattedTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: The Drupal\Tests\codemirror_editor\Kernel\LibraryBuilderTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: Declaring ::setUp without a void return typehint in Drupal\Tests\codemirror_editor\Kernel\LibraryBuilderTest is deprecated in drupal:9.0.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3114724
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: The Drupal\Tests\codemirror_editor\FunctionalJavascript\WidgetStringTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: Declaring ::setUp without a void return typehint in Drupal\Tests\codemirror_editor\FunctionalJavascript\WidgetStringTest is deprecated in drupal:9.0.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3114724
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: The Drupal\Tests\codemirror_editor\FunctionalJavascript\EditorTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: Declaring ::setUp without a void return typehint in Drupal\Tests\codemirror_editor\FunctionalJavascript\WidgetFormattedTest is deprecated in drupal:9.0.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3114724
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: Declaring ::setUp without a void return typehint in Drupal\Tests\codemirror_editor\FunctionalJavascript\FilterTest is deprecated in drupal:9.0.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3114724
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: Declaring ::setUp without a void return typehint in Drupal\Tests\codemirror_editor\FunctionalJavascript\TextEditorTest is deprecated in drupal:9.0.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3114724
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: The Drupal\Tests\codemirror_editor\FunctionalJavascript\SettingsFormTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: The Drupal\Tests\codemirror_editor\FunctionalJavascript\FormatterTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: Declaring ::setUp without a void return typehint in Drupal\Tests\codemirror_editor\FunctionalJavascript\FormatterTest is deprecated in drupal:9.0.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3114724
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: The Drupal\Tests\codemirror_editor\FunctionalJavascript\FilterTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

  1x: The Drupal\Tests\codemirror_editor\FunctionalJavascript\TextEditorTest::$modules property must be declared protected. See https://www.drupal.org/node/2909426
    1x in DrupalListener::startTest from Drupal\Tests\Listeners

Proposed resolution

Address deprecations.

Remaining tasks

Post patch.

User interface changes

None.

API changes

None.

Data model changes

None.