Hi,
Seeds could even be dangerous here, as these numbers are supposed to be 
cryptographically secure. If you need a seedable PRNG for testing, just use 
rand().
Not only it could be dangerous, it would beat the entire purpose of 
random_bytes()/random_int(). Just to clarify for readers not familiar 
with the topic:
Seed-based RNGs are deterministic. 
deterministic === predictable 
predictable === not secure
Whether you want to seed for testing purposes, or someone has beaten 
it into you to use random_*() instead of (mt_)rand() and now your code 
doesn't work the same way, you're likely blindly following best 
practices without consideration. Not everything is 100% testable and 
not every problem can have the same solution.
If you need to generate secure tokens of some kind - use random_bytes(). 
If you need to generate unpredictable random numbers - use random_int(). 
Don't worry about testing either of those. 
If you need seed-based, reproducible outcomes - use mt_rand(), that's 
perfectly fine for e.g. re-creating the same "random" map layout in a 
video game - a valid use case; but it's not for security.
Cheers, 
Andrey.