Remove unused KernelTestBase::getCompiledContainerBuilder()

Problem/Motivation

I'm calling this a bug because when you lift the box, the roaches scurry.

This is all the result of trying to work on deprecation error problems from #2870194: Ensure that process-isolated tests can use Symfony's PHPunit bridge to catch usages of deprecated code

KernelTestBase has two items of interest. Here is the docblock for KernelTestBase::getCompiledContainerBuilder(), which builds a container and stores it for use in all tests.

  /**
   * [...]
   * This method is not invoked if there is only a single test method. It is
   * also not invoked for tests running in process isolation (since each test
   * method runs in a separate process).
   * [...]
   */
  private function getCompiledContainerBuilder(array $modules) {

Note that it says we don't use this precompiled container if the test class is in a separate process.

Now, let's take a look at KernelTestBase::$runTestInSeparateProcess, which it inherits from PHPUnit_Framework_TestCase:

  /**
   * {@inheritdoc}
   *
   * Kernel tests are run in separate processes to prevent collisions between
   * code that may be loaded by tests.
   */
  protected $runTestInSeparateProcess = TRUE;

This forces all tests to be run in separate processes, always.

If we comment out this property (causing all tests to be run in the same process), we end up with assertion errors from ContainerBuilder::__sleep(). That's because getCompiledContainerBuilder() is so vestigial that it breaks when it tries to instantiate a container.

This is because it's never used, since KernelTestBase tests are always process isolated, and because there's no test for it.

If we bypass it, we get failed tests:

$ ./vendor/bin/phpunit -c core/ --testsuite kernel --filter DatabaseBackendTest
PHPUnit 4.8.35 by Sebastian Bergmann and contributors.

Testing 
F..........

Time: 29.45 seconds, Memory: 56.00MB

There was 1 failure:

1) Drupal\KernelTests\Core\Cache\DatabaseBackendTest::testSetGet
Created time is correct.
Failed asserting that false is true.

/Users/paul/pj2/drupal/core/tests/Drupal/KernelTests/AssertLegacyTrait.php:31
/Users/paul/pj2/drupal/core/tests/Drupal/KernelTests/Core/Cache/GenericCacheBackendUnitTestBase.php:158
/Users/paul/pj2/drupal/core/tests/Drupal/KernelTests/Core/Cache/DatabaseBackendTest.php:35

FAILURES!
Tests: 11, Assertions: 136, Failures: 1.

Proposed resolution

Remove getCompiledContainerBuilder().

Deprecate the now-vestigial isTestInIsolation().

Figure out and fix resulting failing tests.

Remaining tasks

User interface changes

API changes

Data model changes