Make sure that the $required param for external_description and its
subclasses are either VALUE_DEFAULT, VALUE_REQUIRED, or VALUE_OPTIONAL.

Some calls to the external_multiple_structure's constructor incorrectly
pass parameters where the value for the $description parameter is not
being passed. This results to invalid values being passed for the
$required parameter.

Replace steps that manually add Book instances via the UI and use
Behat generators. This improves the speed of the Behat test runs.
Additionally, performed Behat cleanup and optimisation to affected
files.

Ensure column and filters provided by each source have sufficient
coverage. Re-factor individual entity/helper class tests to test
their own specific implementation only.

Remove now-defunct test fixtures.

Behat cleanup and optimisation

Replace steps that manually add Wiki instances via the UI and use
Behat generators. This improves the speed of the Behat test runs.

Calls to `str_ends_with` are polyfilled currently, as the method is
not present in PHP7.4 or earlier. We should avoid uses of it so as
to not trip up compatibility standard checkers.

The Behat step to expand all fieldsets did not work if the page
contains only one fieldset.