Drupal 8 doesn't have any schema changes, but there is a placeholder system_update_8000() that was added when the 6.x-7.x updates were removed, that should be enough to add boilerplate tests.
What we need:
1. Update scripts/dump-database-d6.sh and scripts/generate-d6-content.sh to Drupal 7.
2. Create minimal, standard and standard+all optional modules database dumps using 7.0.
3. Add very basic tests that load the databases, run the tests, and maybe do something stupid like visit the front page and makes sure it's still there.
I just ran a clean 7.2 to 8.x upgrade, and amazingly that currently works, so those tests should pass.
Sincewe only support upgrades from the latest stable release, so ideally we'd keep the 7.x database somewhat up-to-date (the absolute best case would be whenever a 7.x point release comes out, but some of these may not contain any updates).
This needs to be in place before any real updates get committed to 8.x.
PASSED: [[SimpleTest]]: [MySQL] 32,969 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 32,980 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 32,968 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 32,978 pass(es).
FAILED: [[SimpleTest]]: [MySQL] 32,957 pass(es), 4 fail(s), and 0 exception(es).