[10.x] Fix $exceptTables to allow an array of table names

[cwilby]

The current implementation of Database Truncation in Laravel has a bug where it requires table names to be grouped by connection name, to ensure that specific tables are not truncated between tests.

An example is shown below, and the current code is here.

abstract class DuskTestCase extends BaseTestCase
{
    use CreatesApplication;
    use DatabaseTruncation;

    protected $connectionsToTruncate = ['mysql_testing'];
    protected $seeder = 'DatabaseSeeder';
    protected $exceptTables = [
        'mysql_testing' => [
            'table1',
            'table2'
        ]
    ];
}

According to the documentation, it should be possible for this array to contain table names, as follows:

abstract class DuskTestCase extends BaseTestCase
{
    use CreatesApplication;
    use DatabaseTruncation;

    protected $connectionsToTruncate = ['mysql_testing'];
    protected $seeder = 'DatabaseSeeder';
    protected $exceptTables = [
        'table1',
        'table2'
    ];
}

This PR changes the DatabaseTruncation trait to allow $exceptTables to be an array of table names.