Remove a laravel passport user token

In my unit test, I have a user for whom I generate a token:

$tokenString = $this->user->createToken('PHPunit', ['example'])->accessToken;

How can I afterward delete this user's token?

This is what I do when a user logged out.

public function logout() {
    Auth::user()->tokens->each(function($token, $key) {
        $token->delete();
    });

    return response()->json('Successfully logged out');
}

This code will remove each token the user generated.