Proposal: Add Testing Environment class

[fubuloubu]

This fixture would be the primary access point for test environment (tester) functionality, which might include:

• [ ] Configuring mining (automine or interval mining)
• [ ] Deploying contracts from packages, or deploying new ones from code
• [ ] Working with time tester.now() e.g. while contract.expiration() > tester.now(): ...
• [ ] Configuring number of accounts and account's ether balances when starting a test
• [ ] Specifying expected failures with tx_fails: ... (and reverting that block)
• [ ] Accessing web3 (for advanced queries of the tester chain)

[fubuloubu]

Being able to manipulate time easily would be nice. For example, let's say you have a contract that allows you to withdraw funds in 7 days (e.g. Plasma). You should be able to do this:

contract.deposit(transact={})
# Can't withdraw a deposit within 7 days
with tx_fails:
    contract.withdraw(transact={})
# Set time to 7 days, minus 1 block
tester.set_time(tester.now() + tester.utils.days(7) - 15)
with tx_fails:
    contract.withdraw(transact={})
# Each block is 15 seconds
tester.mine()
# Now I can withdraw!
contract.withdraw(transact={})