Testnet configurability

[wjmelements]

We want to be able to specify different constants and functions for testnets and devnets. Ideally we don't have to rely on constructor parameters and immutables.

• We want to be able to override IsProbablyASafe to allow devnets and client integration tests to have simpler tests that don't need to deploy Safes.
• We want to override the length of a quarter for devnets and calibnet to allow rapid testing of quarter transitions

[wjmelements]

Related to #5 and #25 in that it relates to scripting and deployment automation.

[BigLep]

Note that there is discussion about whether we should even be asserting "isASafe" per filecoin-project/FIPs#1278 (comment)