Issue #2381763 by chx, webflo: Adjust the order of container yamls to override settings per environment

Issue #2392787 by beejeebus, dawehner, alexpott: Move include statements from DrupalKernel::boot() into DrupalKernel::preHandle()

Issue #2328111 by dawehner, martin107, neclimdul: Replace most instances of the DRUPAL_ROOT constant with the app.root container parameter.

Issue #2367557 by mpdonadio: Fixed Update TestKernel to match class loader changes in Drupal Kernel.

Issue #2028109 by tim.plunkett, almaudoh, twistor, larowlan, slashrsm, Berdir, Arla, neclimdul, Cottser: Convert hook_stream_wrappers() to tagged services.

Issue #2248845 by sun: Fixed [run-tests.sh] simpletest_phpunit_xml_filepath() depends on public:// stream wrapper.

Issue #2016629 by larowlan, neclimdul, sun, alexpott, jibran, ParisLiakos, donquixote, effulgentsia, msonnabaum: Refactor bootstrap to better utilize the kernel.

Revert "Issue #2016629 by larowlan, neclimdul, sun, alexpott, jibran, ParisLiakos, donquixote, effulgentsia, msonnabaum: Refactor bootstrap to better utilize the kernel."

This reverts commit cda051c4.

Issue #2016629 by larowlan, neclimdul, sun, alexpott, jibran, ParisLiakos, donquixote, effulgentsia, msonnabaum: Refactor bootstrap to better utilize the kernel.

Issue #2168809 by sun, damiankloip: Do not add the Guzzle HTTP client request subscriber outside of test environment.

Issue #2257427 by sun: Run-tests.sh builds + rebuilds two kernels from scratch for every PHP process.