[TESTS] Add some basic tests for ModelsCommand (#869)

* tests: add abstract class for all database related tests

* tests: add abstract class for all ModelsCommand related tests

* tests: add GenerateBasicPhpdoc test

* tests: add CustomDate test

* tests: remove MITM abstract TestCaseDatabase, not useful here

* tests: skip new ModelsCommand test on Laravel < 6

* tests: remove CarbonImmutable checked

Since we decided to skip for Laravel < 6.0 anyway, it's always present

tests/Console/ModelsCommand/GenerateBasicPhpdoc/Models/Post.php

@@ -0,0 +1,9 @@
+<?php declare(strict_types=1);
+
+namespace Barryvdh\LaravelIdeHelper\Tests\Console\ModelsCommand\GenerateBasicPhpdoc\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+class Post extends Model
+{
+}