Problems with Oracle 21 (phpunit testing failures)

For Moodle 4.1 Oracle requirements were raised from 11g to 19c (given support timeframes... see MDL-71747 for more info).

So, we created a new moodle-db-oracle:21 image to be used by CIs and other tools. Oracle 21c was picked, instead of 19c, because it has official XE images and offered better compatibility with the pre-existing (also XE) 11g image.

Some initial benchmarking has shown that the 21 image is noticeably slower than the 11 one (say 25%, not accurate number!) and this may impact some of the failures we are getting. But not all!

Here there are some recent runs, using the Oracle 21c image:

• PHPUnit: https://ci.moodle.org/view/Testing/job/DEV.02%20-%20Developer-requested%20PHPUnit/7556/
• Behat (non-js): https://ci.moodle.org/view/Testing/job/DEV.01%20-%20Developer-requested%20Behat/25247/
• Behat (js): https://ci.moodle.org/view/Testing/job/DEV.01%20-%20Developer-requested%20Behat/25246/

This issue is about to investigate how we can get them back to pass, analysing the causes for all the failures. So, some su-issues should be created about each individual failure type.

Also, maybe the images themselves need some improvements (that could impact positively to results) and efforts will be done in that area too.

Ultimately, if this takes too long, we can, in an interim way, skip some of those tests to ensure that everything else continues working ok with Oracle. Till then, we'll get near-consistent failures with Oracle in master (4.1dev).