Different database in different tests - is it possible?

Can you not just override the ENV variables to change the database?

Yes you can. Setup a new connection in config/database.php and override the DB_CONNECTION env variable in phpunit.xml in the configuration section. I think in this lesson Jeffrey goes through the exact steps to create an sqlite testing database. If it isn't that lesson it is one of them in that series.

Where do you think would be the best place to do this? In the test setUp() method?

@smnhunt The seeInDatabase function also has accepts an extra parameter for a database connection

protected function seeInDatabase($table, array $data, $connection = null)

Not sure what you want to test. But I think you can do something with this as well ;)

For example

public function setUp()
{
    $this->connection = 'sqlite-testing';
}

public function test_something()
{
    $this->seeInDatabase('users', [
        'id' => 1,
    ], $this->connection);
}