Further document Build and in general improve testing docs

Problem/Motivation

As discovered in #3086373: Build test framework cleanup: bootstrap and tests, we are missing docs. Lets add them.

core.api.php could use an update:
https://git.drupalcode.org/project/drupal/blob/8.8.x/core/core.api.php#L... for example.

Same with core/tests/README.md:
https://git.drupalcode.org/project/drupal/blob/8.8.x/core/tests/README.md

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet