Common: Convert system functional tests to phpunit

I'd say this is system module since the tests live the there. Wondering if tests in system.module should instead be moved to core/tests instead, we only had them in system.module because that was the only place they could be before..

Some hints on fixing the tests

* I think instead of setRawContent(), you want to use assertContains() or similar assertions on the string that you have.
* assertNoCacheTag() and assertCacheTag() should possibly be copied into some cache asserts trait?
* settingsSet(): Looks like we have no replacement for that yet in browser test base, \Drupal\Tests\system\Functional\System\SitesDirectoryHardeningTest::testSitesDirectoryHardeningConfig() inlines it, but we do have \Drupal\KernelTests\KernelTestBase::setSetting(), maybe that could move into common trait or something.
* assertNoPattern() is in AssertContentTrait
* assertCacheContext() is in \Drupal\system\Tests\Cache\AssertPageCacheContextsAndTagsTrait, sounds like a nice place for assertCacheTag methods above?

Not sure what to do with Drupal\Tests\system\Functional\Common\SimpleTestErrorCollectorTest::testErrorCollect(). That's actually a simpletest test, so we might not want to remove that until we remove simpletest and instead copy it to be able to test the same with BrowserTestBase if necessary, which might look quite different.

Note: #2955690: Move Common tests in system.module to BTB contains a bunch of tests which just required a 1to1 movement

Here we go, no interdiff, because this is quite different then the moves from #2

• Converted the stuff that I could to a Kerneltest
• Moved SimpleTestErrorCollectorTest to the simpletest module and tagged it with WebTestBase group, that can be removed when WebTestBase gets removed
• Unclear what setSetting was doing, since the settings were never used, just set up in setUp(), so just removed that (am I overlooking something?).

1. +++ b/core/modules/system/tests/src/Functional/Common/FormatDateTest.php
   @@ -38,11 +38,6 @@ protected function setUp() {
        $this->refreshVariables();
    
   -    $this->settingsSet('locale_custom_strings_' . self::LANGCODE, [
   -      '' => ['Sunday' => 'domingo'],
   -      'Long month name' => ['March' => 'marzo'],
   -    ]);
   -
        ConfigurableLanguage::createFromLangcode(static::LANGCODE)->save();
        $this->resetAll();
   

   This is not longer needed as #2795601: Convert \Drupal\Tests\system\Functional\Common\FormatDateTest mostly to a kernel test moved almost everything over

2. +++ b/core/modules/system/tests/src/Kernel/Common/FormElementsRenderTest.php
   @@ -162,8 +140,9 @@ public function testDrupalRenderFormElements() {
      protected function assertRenderedElement(array $element, $xpath, array $xpath_args = []) {
   -    $original_element = $element;
   -    $this->setRawContent(drupal_render_root($element));
   +    /** @var \Drupal\Core\Render\RendererInterface $renderer */
   +    $renderer = $this->container->get('renderer');
   +    $this->setRawContent($renderer->renderRoot($element));
   

   Is there a reason we are not just using $this->render() ?

@dawehner++

12.1 removed a whole bunch of stuff
12.2 nope, no reason other then me forgetting that that is a thing :)

Also fixed the namespace for the moved simpletest (duh)

This looks very good, great work! I have no suggestions or nits to pick.

Updating issue credit

Committed 17da127 and pushed to 8.6.x. Thanks!

diff --git a/core/modules/system/tests/src/Functional/Common/FormatDateTest.php b/core/modules/system/tests/src/Functional/Common/FormatDateTest.php
index fe22c134e8..062d6f4124 100644
--- a/core/modules/system/tests/src/Functional/Common/FormatDateTest.php
+++ b/core/modules/system/tests/src/Functional/Common/FormatDateTest.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Tests\system\Functional\Common;
 
-use Drupal\language\Entity\ConfigurableLanguage;
 use Drupal\Tests\BrowserTestBase;
 
 /**

Fixing unused use on commit.