Here is a status report after the initial 5 hours:
- reviewed the last pre-existing issue (https://www.drupal.org/node/2428695), fixed a few things and it's ready to go
- decided to keep the scope of the original meta (https://www.drupal.org/node/2318191) focused on the Database tests and marked it as resolved
- identified all the remaining test fails and opened a new meta issue to track them: https://www.drupal.org/node/2454513
- started working on them by giving a priority to those that could be fixed together
- posted a complete patch in https://www.drupal.org/node/2454625 which fixes 7 test classes
- posted an initial patch in https://www.drupal.org/node/2454669 which is the first step for fixing the tests in the migrate_drupal group; the second part of the problem is not very straightforward and needs further investigation, this is actually where almost half of the time was spent
The new meta issue (https://www.drupal.org/node/2454513) provides a clear overview (I hope) of the work required to have all the tests fully passing on SQLite. As far as the estimate for completing all the tasks, I think my initial estimation, 1 to 4 days, was mostly correct.
Although, after seeing what kind of problems we can bump into, I would revise it to 2 to 5 full days, mostly because the remaining tests that need to be fixed are some of the most long-running ones (just InstallUninstallTest takes around 6 minutes to run and ConfigImportAllTest takes 4 minutes). For the moment, the high estimation also includes further weekly monitoring of the temporary SQLite testbot and fixing potential new issues, until the DrupalCI team can provide a permanent solution for this so we can detect problems more quickly.