services.yml

services:
  my.something:
    class: My\Something
    # arguments:
    #  - @...
    
  my.another_thing:
    class: My\AnotherThing
    

ServiceTest.php

<?php

use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;

class ServiceTest extends KernelTestCase
{
    public function provideExpectedDefinitions()
    {
        return [
            ['My\Something', 'my.something'],
            ['My\AnotherThing', 'my.another_thing'],
        ];
    }

    /**
     * @test
     * @group functional
     * @dataProvider provideExpectedDefinitions
     */
    protected function it_registers_declared_services_correctly($class, $serviceId)
    {
        $kernel = static::createKernel();

        static::assertInstanceOf($class, $kernel->getContainer()->get($serviceId));
    }
}