Move logic out of ApiContentUpdateForDemoControllerTest into more other tests

Asking for @wim leers review again. The test cases are being moved to

1. \Drupal\Tests\experience_builder\Kernel\ClientServerConversionTraitTest::testConvertClientToServerErrors
2. or \Drupal\Tests\experience_builder\Kernel\ClientDataToEntityConverterTest::testConvert if the errors are caught in \Drupal\experience_builder\Controller\ClientServerConversionTrait::convertClientToServer but only caught in \Drupal\experience_builder\ClientDataToEntityConverter::convert

\Drupal\Tests\experience_builder\Functional\ApiContentUpdateForDemoControllerTest::testSave is still left with test cases that prove that if the call to
$violations = $this->clientDataToEntityConverter->convert(json_decode($request->getContent(), TRUE), $entity);

gets an error entity will not be saved

I'd love to see @larowlan approve this too, or suggest an alternative 🤞

I think this controller is temporary code so this unblocks more work

Thanks @wim leers and @larowlan!