[#351] [#53] Add `getNow` and `advanceTime` to Morley.Nettest

Description

Problem: sometimes, we might want to test scenarios where we need to advance time by some given number of seconds.

We've needed this in stablecoin and tezos-nbit before, and we're seeing the need for this again in BaseDAO.

https://github.com/tqtezos/stablecoin/blob/master/haskell/test/Lorentz/Contracts/Test/Permit.hs#L151-L160

Additionally, in tezos-nbit we've also needed to know what the current time is. In Integrational, we can just use dummyNow, but there's no way to do this in Morley.Nettest at the moment.

Solution: Add getNow and advanceTime to Morley.Nettest.

Related issue(s)

Resolves #351 (closed) Resolves part of #53 (closed)

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