Refactor InstallerNonDefaultDatabaseDriverTest to avoid asserting content of settings.php

From https://www.php.net/manual/en/function.include.php

When a file is included, the code it contains inherits the variable scope of the line on which the include occurs. Any variables available at that line in the calling file will be available within the called file, from that point forward. However, all functions and classes defined in the included file have the global scope.

So in this case, $databases is a local variable of InstallerNonDefaultDatabaseDriverTest::getInstalledDatabaseSettings() coming from the evaluation of the code in the included settings.php file.

I had to add the assert since PHPStan was complaining more or less for the same reason of your comment - PHPStan cannot determine what's included in the include (pun intended), so we need to inform it that a variable with that name is existing at that point.

Once the parent is committed I will rebase and add a comment.

Done.