How to generate arbitrary wallet seeded with eth in hardhat tests using ethers.js?

Hardhat allows to seed a custom amount of prefunded addresses using the accounts.count config option of the built-in hardhat network.

Example:

module.exports = {
    networks: {
        hardhat: {
            accounts: {
                count: 1000
            }
        }
    }
}

Docs: https://hardhat.org/hardhat-network/reference/#config

The new ethers.Wallet.createRandom() function only creates an account from a random private key in the context of the JS application, but it doesn't communicate with the provider (Hardhat in your case).

I think you'll probably want to use the hardhat network method hardhat_setBalance, the docs use an example like this:

await network.provider.send("hardhat_setBalance", [
  "<ACCOUNT ADDRESS>",
  "0x1000", # 4096 wei
]);

I'm not developing in javascript though, but I've been doing something similar in python using web3.py and eth-account with code like this:

from web3 import Web3
from eth_account import Account


chain = Web3(HTTPProvider("http://127.0.0.1:8545"))

acct = Account.create('<RANDOM VALUE>')
address = Web3.toChecksumAddress(acct.address)
print(chain.eth.get_balance(address)) # 0 wei

# add a balance of eth tokens to the address
chain.provider.make_request("hardhat_setBalance", [address, "0x1000"])

print(chain.eth.get_balance(address)) # 4096 wei