[9.x] Adds "PendingCommand::assertOk()"

[nunomaduro]

While creating tests for Artisan commands, I've noticed that assertSuccessful is just too many characters for me. So, this pull request proposes an assertOk() method that is simply an alias of assertExitCode(0):

it('can run', function () {
    $this->artisan('inspire')->assertOk();
});

Note that, this combines well with the HTTP layer regarding testing, as we we already have an assertOk() there:

$this->get('/')->assertOk();