Deprecate simpletest within DrupalKernel (DRUPAL_TEST_IN_CHILD_SITE)

Problem/Motivation

Bet you didn't know DrupalKernel has Simpletest in it.

DrupalKernel::bootEnvironment() says this:

    // Indicate that code is operating in a test child site.
    if (!defined('DRUPAL_TEST_IN_CHILD_SITE')) {
      if ($test_prefix = drupal_valid_test_ua()) {
        $test_db = new TestDatabase($test_prefix);
        // Only code that interfaces directly with tests should rely on this
        // constant; e.g., the error/exception handler conditionally adds further
        // error information into HTTP response headers that are consumed by
        // Simpletest's internal browser.
        define('DRUPAL_TEST_IN_CHILD_SITE', TRUE);

        // Web tests are to be conducted with runtime assertions active.
        assert_options(ASSERT_ACTIVE, TRUE);
        // Now synchronize PHP 5 and 7's handling of assertions as much as
        // possible.
        Handle::register();

        // Log fatal errors to the test site directory.
        ini_set('log_errors', 1);
        ini_set('error_log', $app_root . '/' . $test_db->getTestSitePath() . '/error.log');

        // Ensure that a rewritten settings.php is used if opcache is on.
        ini_set('opcache.validate_timestamps', 'on');
        ini_set('opcache.revalidate_freq', 0);
      }
      else {
        // Ensure that no other code defines this.
        define('DRUPAL_TEST_IN_CHILD_SITE', FALSE);
      }
    }

This code fragment exists so that we can test WebTestBase.

When we eventually deprecate TestBase/WebTestBase, we won't be able to deprecate DrupalKernel::bootEnvironment().

Proposed resolution

Factor this code out of bootEnvironment() and into a separate static method which can be deprecated.

Deprecate the new method.

Somehow formally deprecate usages of DRUPAL_TEST_IN_CHILD_SITE which exists solely to isolate TestBase and WebTestBase from themselves during testing.

Remaining tasks

Figure out how to deprecate drupal_valid_test_ua() and friends.

User interface changes

API changes

Data model changes