Investigate PHP 7.3 workarounds

Problem/Motivation

#2989734: PHP 7.3 compatibility → made Drupal 8 pass on PHP 7.3 but we introduced some workarounds to overcome differences in PHP 7.3.

The workarounds are:

• Drupal\KernelTests\Core\Entity\EntityQueryTest does not use list()
• PrivateTempStore had to use DependencySerializationTrait - whilst the fix is correct we should work out what has changed in PHP's serialize()
• BrowserTestBase::getDrupalSettings() had to change to use $this->getSession()->getPage()->getContent() instead of $this->getSession()->getPage()->getHtml() in order for it to work

Proposed resolution

Fix the tests or work out why this is happening.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None