I have run many tests and now accounts[0]
has run out of ETH in Ganache.
Easiest way to get more ETH,
so I can start running tests again ?
If I understand correctly, you should be able to find a TestEth faucet for whichever testnet you are running.
Maybe you wanna look into this post, there are some faucets for testnets.
Hope you find this useful.
If you have any more questions, please let us know so we can help you! 
Carlos Z.