When a cachable object is store in the static cache from
the backing store, it was incorrect serialised rather than
using the wake function.  This has been resolved and tests added.

During the investigation into cacheable_object, it was discovered
that set_identifiers never removes identifiers when you call it,
so set_identifiers(array('a')) and set_identifiers(array('b')) really
resulted in array('a','b') as the identifiers rather than 'b'.

The fix for this issue depends on the set_identifiers fix and
they have been coupled together as a result.

if theme/tests/behat/behat_theme_{THEME_NAME}_behat_selectors.php
file exits, and has class with same file name then we will
call behat_theme_{THEME_NAME}_behat_selectors::register_moodle_selectors()
to register theme specific selectors. If they are different
then core then they will override

This isn't used in moodle core and is problematic on oracle, don't test
it.

If error is not displayed in web browser then
it will be caught in shutdown hook, so we can
show it to user

The completion cache is currently not marked as simpledata. On the
course page it is frequently retrieved hundreds of times which results
in many calls to the slow unserialize function. By making a slight
change to the data format (using arrays instead of objects) we can
mark it as simpledata, which will avoid using unserialize.

If the user cannot subscribe, there is no point showing the digest options.

Daily digest options should not be generated if the user cannot subscribe
to anything in the first place.