Mailable.php

<?php

namespace App\Mail;

use Illuminate\Container\Container;
use Illuminate\Mail\Mailable as BaseMailable;
use PHPUnit\Framework\Assert;

class Mailable extends BaseMailable
{
    /**
     * @return array
     */
    public function getRenderedBody() : array
    {
        Container::getInstance()->call([$this, 'build']);

        return $this->buildView();
    }

    /**
     * @param string $string
     *
     * @return $this
     */
    public function assertHtmlBodyContains(string $string)
    {
        Assert::assertTrue(
            str_contains($this->getRenderedBody()['html'], $string),
            "The rendered HTML body does not contain \"$string\"."
        );

        return $this;
    }

    /**
     * @param string $string
     *
     * @return $this
     */
    public function assertTextBodyContains(string $string)
    {
        Assert::assertTrue(
            str_contains($this->getRenderedBody()['text'], $string),
            "The rendered text body does not contain \"$string\"."
        );

        return $this;
    }
}

Test.php

<?php

public function testItCanMail()
{
    Mail::assertSent(MyMail::class, function (MyMail $mail) use ($url) {
        return $mail->assertHtmlBodyContains($url)
                    ->assertTextBodyContains($url)
                    ->hasTo(config('app.mail_to'));
    });
}