I broke down each separate implementation into their files; running details at the end of the file

testingTask.php

//BaseTest.php file name (same convention is followed)

<?php

use PHPUnit\Framework\TestCase;
include'./Hero.php'; // i utilized includes to be swift

class BaseTest extends TestCase
{
    public $defender;
    public $attacker;
    public $fightService;

    const HEALTH = 50;
    const ATTACK_ONE = 50;
    const DEFENSE_ONE = 100;
    const ATTACK_TWO = 50;
    const DEFENSE_TWO = 30;
    const UPDATED_HEALTH = 50;

    protected function setUp(): void
    {
        parent::setUp();

        $this->attacker = new Hero(self::ATTACK_ONE,self::DEFENSE_ONE,self::HEALTH);
        $this->defender = new Hero(self::ATTACK_TWO,self::DEFENSE_TWO,self::HEALTH);
        $this->fightService = new FightService;
    }
}

//FightServiceTest.php

<?php

include './BaseTest.php';

class FightServiceTest extends BaseTest {


    public function testFight(): void
    {
        $this->fightService->fight($this->attacker,$this->defender);

        $this->assertNotEquals(self::HEALTH, $this->defender->getHealthPoints());
        $this->assertEquals(self::UPDATED_HEALTH, $this->attacker->getHealthPoints());
    }
}


//HeroTest.php

<?php

include './BaseTest.php';

class HeroTest extends BaseTest
{

    public function testAttackAmount()
    {
        $this->assertEquals(self::ATTACK_ONE, $this->attacker->getAttack());
    }

    public function testDefenseAmount()
    {
        $this->assertEquals(self::DEFENSE_ONE, $this->attacker->getDefence());
    }

    public function testHealthPoint()
    {
        $this->assertEquals(self::HEALTH, $this->attacker->getHealthPoints());
    }

    public function testCanUpdateHealthPoint()
    {
        $this->assertEquals(self::HEALTH, $this->attacker->getHealthPoints());
        $this->attacker->setHealthPoints(self::UPDATED_HEALTH);
        $this->assertEquals(self::UPDATED_HEALTH, $this->attacker->getHealthPoints());
    }
}

//Hero.php (content of this file can be further broken down but i didnt do that here)

<?php

interface HeroInterface
{
    public function getAttack(): int;

    public function getDefence(): int;

    public function getHealthPoints(): int;

    public function setHealthPoints(int $healthPoints);
}

class Hero implements HeroInterface
{

    private int $attack;
    private int $defence;
    private int $healthPoint;

    /**
     * @param int $attack
     * @param int $defence
     * @param int $healthPoint
     */
    public function __construct(int $attack, int $defence, int $healthPoint)
    {
        $this->attack = $attack;
        $this->defence = $defence;
        $this->healthPoint = $healthPoint;
    }


    /**
     * @return int
     */
    public function getAttack(): int
    {
        return $this->attack;
    }

    /**
     * @return int
     */
    public function getDefence(): int
    {
        return $this->defence;
    }

    /**
     * @return int
     */
    public function getHealthPoints(): int
    {
        return $this->healthPoint;
    }

    /**
     * @param int $healthPoints
     * @return mixed
     */
    public function setHealthPoints(int $healthPoints): void
    {
        $this->healthPoint = $healthPoints;
    }
}

class DamageCalculator
{
    const DAMAGE_RAND_FACTOR = 0.2;

    public static function calculateDamage(HeroInterface $attacker, HeroInterface $defender): int
    {
        $damage = 0;

        if ($attacker->getAttack() > $defender->getDefence()) {
            $baseDamage = $attacker->getAttack() - $defender->getDefence();

            $factor = $baseDamage * self::DAMAGE_RAND_FACTOR;

            $minDamage = $baseDamage - $factor;
            $maxDamage = $baseDamage + $factor;

            $damage = mt_rand($minDamage, $maxDamage);
        }

        return $damage;
    }
}

class FightService
{
    public function fight(HeroInterface $attacker, HeroInterface $defender)
    {
        $damage = DamageCalculator::calculateDamage($attacker, $defender);

        $defender->setHealthPoints($defender->getHealthPoints() - $damage);
    }
}


To run:
ensure php unit is globally available or use composer to pull it in from packagist (i used composer)
Unit Test for Hero: ./vendor/bin/phpunit  HeroTest.php --color
Feature Test for FightService:  ./vendor/bin/phpunit  FightServiceTest.php --color