[DOC] Missing info on workspaces-js testing

[flmel]

Is your feature request related to a problem? Please describe.
Users had issue when trying to test their contract on testnet. Issue was related to setting up and creating accounts via the worker.

Describe the solution you'd like
@gagdiez shared info from previous experience

// init the worker like this
  const worker = t.context.worker = await Worker.init({
    network: 'testnet',
    testnetMasterAccountId: '<your account>.testnet',
    initialBalance: NEAR.parse("<X> N").toString(),
  });

and you need to create the file:

.near-credentials/workspaces/testnet/<your-account>.testnet.json with the credentials
eg.

{"account_id":"guille.testnet","public_key":"ed25519:GnsdHdSrhe8v3ssAdie3RuXy59tMredkwRAFZ9xeWxSWF","private_key":"ed25519:..."}

example snippet

 // Create accounts
  const root = worker.rootAccount;

  const alice = await root.createSubAccount("alice", { initialBalance: NEAR.parse("10 N").toString() });
  const bob = await root.createSubAccount("bob", { initialBalance: NEAR.parse("10 N").toString() });
  const auctioneer = await root.createSubAccount("auctioneer", { initialBalance: NEAR.parse("10 N").toString() });
  const contract = await root.createSubAccount("contract", { initialBalance: NEAR.parse("10 N").toString() });