Try to speed up InstallUninstallTest / ConfigImportAllTest

Problem/Motivation

\Drupal\config\Tests\ConfigImportAllTest
and
\Drupal\system\Tests\Module\InstallUninstallTest
are one of the two slowest tests in Drupal, if not the slowest.

Let's try to figure out whether we can speed them up a bit.

Proposed resolution

Remaining tasks

User interface changes

API changes