Start indexing test accounts with 1

[nkuba]

We should pull accounts in tests starting with index 1 instead of 0.
The 0 is usually a deployer. Using the same account as different roles in tests may cause some false-positive test results.

What would be even better we should use getUnnamedAccounts from hardhat to pull the accounts, so they don't overlap with the named accounts defined in the hardhat config. Please see example.

Originated in: #33 (comment)