Fix all PluginExistsConstraint constraint violations in tests

Problem/Motivation

Discovered in #3361534: KernelTestBase::$strictConfigSchema = TRUE and BrowserTestBase::$strictConfigSchema = TRUE do not actually strictly validate.

For example:

Drupal\Tests\big_pipe\Kernel\BigPipeInterfacePreviewThemeSuggestionsTest::testBigPipeThemeHookSuggestions
Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for block.block.test_block1 with the following errors: 0 [plugin] The 'test_html' plugin does not exist.

Steps to reproduce

Run Drupal core's test suite with #3361534: KernelTestBase::$strictConfigSchema = TRUE and BrowserTestBase::$strictConfigSchema = TRUE do not actually strictly validate applied, and with the lines

      // @see \Drupal\Core\Plugin\Plugin\Validation\Constraint\PluginExistsConstraint::$unknownPluginMessage
      // @todo Remove this in https://www.drupal.org/project/drupal/issues/3362457
      "The '.*' plugin does not exist.",

removed from \Drupal\Core\Config\Schema\SchemaCheckTrait::checkConfigSchema().

Proposed resolution

Solution for the example above:

diff --git a/core/modules/big_pipe/tests/src/Kernel/BigPipeInterfacePreviewThemeSuggestionsTest.php b/core/modules/big_pipe/tests/src/Kernel/BigPipeInterfacePreviewThemeSuggestionsTest.php
index 9064a6074a..928692d006 100644
--- a/core/modules/big_pipe/tests/src/Kernel/BigPipeInterfacePreviewThemeSuggestionsTest.php
+++ b/core/modules/big_pipe/tests/src/Kernel/BigPipeInterfacePreviewThemeSuggestionsTest.php
@@ -60,7 +60,7 @@ public function testBigPipeThemeHookSuggestions() {
     $entity = $this->controller->create([
       'id' => 'test_block1',
       'theme' => 'stark',
-      'plugin' => 'test_html',
+      'plugin' => 'system_powered_by_block',
     ]);
     $entity->save();
 

Remaining tasks

Fix all of these failures.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

N/A