[11.x] Allows asserting no output in Artisan commands

[nunomaduro]

This pull request allows to assert that Artisan commands don't provide any output:

test('backups', function () {
    $this->artisan('backup')
            ->doesntExpectOutput()
            ->assertExitCode(0);
});

At the same time, for feature parity, you may test that a command outputs something:

test('backups', function () {
    $this->artisan('say:hello') // outputs "hello"
            ->expectsOutput()
            ->assertExitCode(0);
});