Drupal 9 compatibility

Here is a patch that update the tests to make them compatible with Drupal 9, as they passed I removed the core: 8.x.

There is just this line $accountProxy = new AccountProxy(); where I'm not 100% sure that my change is correct so it needs an extra look at. I used the code found in AccountProxyTest.php to add the EventDispatcher :

$dispatcher = $this->prophesize(EventDispatcherInterface::class);
$accountProxy = new AccountProxy($dispatcher->reveal());

Missing tag

So the default tests are falling because they try to run Drupal 9 with MySQL 5.5 when the default version is 5.7.

I fixed the remaining failing tests mainly due to PHPUnit deprecated messages.

PS : this needs to be run also with dev version of term_reference_change, so locally this pass :

$ sudo -u www-data ../../vendor/bin/phpunit --group term_merge
PHPUnit 8.5.5 by Sebastian Bergmann and contributors.

Testing
.......................................................           55 / 55 (100%)

Time: 3.97 minutes, Memory: 929.00 MB

OK (55 tests, 437 assertions)