Add adjustTime to FrozenClock

[lgrossi]

Minor helper that simplifies FrozenClock usage during test setup, allowing to modify the clock based on a given modifier.

Benefit:

// instead of
$this->clock->setTo($this->clock->now()->modify('+60 seconds'))

// we can do
$this->clock->adjustTime('+60 seconds');

[lcobucci]

@lgrossi thanks!

[lcobucci]

I just noticed that I didn't update this 🤦
Silly me...