[ETHEREUM-CONTRACTS] Add decimals as an option to deploy-test-token.js

[0xdavinchee]

Summary

Currently our deploy-test-token.js script hardcodes the number of decimals to 18 when deploying the TestToken contract.
However, @kasparkallas brought up a good point which is that there are many ERC20 tokens in the wild which are part of our resolver and prevalent in crypto (USDC) which have a different number of decimals.
Allowing the deployment of test token with a custom number of decimals is going to be useful for testing logic along the entire stack (contracts to client) and we should consider redeploying the fUSDC tokens in the testnets which won't be deprecated with the correct number of decimals (6) so we can properly test this logic internally as well as allowing external devs building on top of Superfluid to have a more realistic testing experience.

TODO

• Allow another property to be included when running deploy-test-token.js: TOKEN_DECIMALS, if it is left empty, default to the standard 18
• (OPTIONAL) Redeploy fUSDC with 6 decimals and remove the current fUSDC from the resolver and add this one to the resolver