Code cleanup from update_fix_compatibility() fix

Here's what I removed from my review of the backport.

1. +++ b/core/modules/system/system.install
   @@ -839,6 +888,115 @@ function system_requirements($phase) {
   +    $is_missing_extension = function ($extension_name) use (&$extension_list) {
   ...
   +    $extension_list = \Drupal::service('extension.list.theme');
   

   Why is $extension_list by reference?
   Ohhh no, is that doing what I think it's doing? Is that so it can reuse the anonymous function but with two different objects?
   Ew.

   Let's not do that, please. It's not worth the DRY effort when it's that tricky and involves references.

2. +++ b/core/modules/system/tests/src/Functional/Update/UpdateScriptTest.php
   @@ -161,6 +172,227 @@ public function testRequirements() {
   +   * Date provider for testExtensionCompatibilityChange().
   

   s/Date/Data

3. +++ b/core/modules/system/tests/src/Functional/Update/UpdateScriptTest.php
   @@ -426,4 +658,69 @@ public function getSystemSchema() {
   +    $extension_config = $this->container->get('config.factory')->getEditable('core.extension');
   

   Not a big deal, but why getEditable? Just catches my eye when I see that without a save()

4. +++ b/core/modules/system/tests/src/Functional/Update/UpdateScriptTest.php
   @@ -426,4 +658,69 @@ public function getSystemSchema() {
   +    $assert_session = $this->assertSession();
   ...
   +    $this->assertSession()->pageTextNotContains($unexpected_error_text);
   ...
   +    $assert_session->pageTextContains('No pending updates.');
   

   Either use the local variable (+1) or don't, but not both.

5. +++ b/core/modules/system/tests/src/Functional/Update/UpdateScriptTest.php
   @@ -426,4 +658,69 @@ public function getSystemSchema() {
   +    $this->clickLink(t('Continue'));
   

   Nit: not t() in tests like this

6. +++ b/core/modules/system/tests/src/Functional/Update/UpdateScriptTest.php
   @@ -426,4 +658,69 @@ public function getSystemSchema() {
   +   * @throws \Behat\Mink\Exception\ExpectationException
   +   * @throws \Behat\Mink\Exception\ResponseTextException
   ...
   +  protected function assertErrorOnUpdate($expected_error_text, $extension_type, $extension_machine_name) {
   

   Don't usually put these on our assert helpers.

7. +++ b/core/modules/system/tests/src/Functional/Update/UpdateScriptTest.php
   @@ -426,4 +658,69 @@ public function getSystemSchema() {
   +    for ($reload = 0; $reload <= 1; $reload++) {
   

   Nittiest of nits, but
   for ($i = 0; $i < 2; $i++) {
   is a lot more legible IMO. Then I know that $reload isn't a magic number, and also I can tell it runs twice instead of thinking about it.

8. +++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
   @@ -206,9 +206,77 @@ public function testReset() {
   +   * DataProvider for testCheckIncompatibility().
   

   Let's be consistent with the others: "Data provider for ..."