testnet3 and 4 have a rule: when the 'nTime' of a block is larger then the nTime of the last block + 20 minutes, the difficulty drops to 1. This means that anyone (cpu/gpu/asic) can easily mine it, because the difficulty is 1.

So you can construct a block with a 'fake' nTime in the future (at least 20 minutes more then the last block), and instantly mine it.

There is a limit to this (else you can keep moving nTime more and more in the future).
The limit is another rule: nTime of block cannot be later then last block + 2 hours.

So it's possible to always mine 5 blocks into the future. This is being done a lot on testnet (because it's an easy way to get some coins without waiting to actually mine it).
 

 You can easily see this on mempool. All the latest block show "Just now", but when you click on them, and view their timestamp, they are often in the future (with a maximum of +2 hours)