BrowserTestBase and KernelTestBase tests break badly when they fail

Problem/Motivation

If you do something like:

    $this->assertEquals('/a-url-that-does-not-match', $this->getSession()->getCurrentUrl());

This will populate BrowserTestBase::$mink and BrowserTestBase::$container, neither or which can be serialised. And the PHPUnit result will contain a stack trace which include the test object which will fail when serialized as part of the way tests in separate processes are run.

Steps to reproduce

1. Add $this->assertSame('asd', 'a'); as the first line of BrowserTestBaseTest::testGoTo()
2. Step up the environmental variables so that tests work
3. phpunit -c core core/modules/simpletest/tests/src/Functional/BrowserTestBaseTest.php

Expected result

1) Drupal\Tests\simpletest\Functional\BrowserTestBaseTest::testGoTo
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-asd
+a

/Volumes/devdisk/dev/sites/drupal8alt.dev/vendor/phpunit/phpunit/src/Framework/Constraint.php:110
/Volumes/devdisk/dev/sites/drupal8alt.dev/vendor/phpunit/phpunit/src/Framework/Constraint/IsIdentical.php:88
/Volumes/devdisk/dev/sites/drupal8alt.dev/vendor/phpunit/phpunit/src/Framework/Assert.php:2183
/Volumes/devdisk/dev/sites/drupal8alt.dev/vendor/phpunit/phpunit/src/Framework/Assert.php:1174
/Volumes/devdisk/dev/sites/drupal8alt.dev/core/modules/simpletest/tests/src/Functional/BrowserTestBaseTest.php:30
/Volumes/devdisk/dev/sites/drupal8alt.dev/vendor/phpunit/phpunit/src/Framework/TestCase.php:889
/Volumes/devdisk/dev/sites/drupal8alt.dev/core/modules/simpletest/src/BrowserTestBase.php:822
/Volumes/devdisk/dev/sites/drupal8alt.dev/vendor/phpunit/phpunit/src/Framework/TestCase.php:749
/Volumes/devdisk/dev/sites/drupal8alt.dev/vendor/phpunit/phpunit/src/Framework/TestResult.php:601
/Volumes/devdisk/dev/sites/drupal8alt.dev/vendor/phpunit/phpunit/src/Framework/TestCase.php:705

Actual result

1) Drupal\Tests\simpletest\Functional\BrowserTestBaseTest::testGoTo
PHPUnit_Framework_Exception: PHP Fatal error:  Uncaught exception 'LogicException' with message 'The database connection is not serializable. This probably means you are serializing an object that has an indirect reference to the database connection. Adjust your code so that is not necessary. Alternatively, look at DependencySerializationTrait as a temporary solution.' in /Volumes/devdisk/dev/sites/drupal8alt.dev/core/lib/Drupal/Core/Database/Connection.php:1433
Stack trace:
#0 [internal function]: Drupal\Core\Database\Connection->__sleep()
#1 -(70): serialize(Array)
#2 -(83): __phpunit_run_isolated_test()
#3 {main}
  thrown in /Volumes/devdisk/dev/sites/drupal8alt.dev/core/lib/Drupal/Core/Database/Connection.php on line 1433

The same problem afflicts KernelTestBase tests see #6

Eligible for 8.0.x because it is a test only change.

Proposed resolution

Don't serialise any properties - there is no point.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None