[PROD-2020] Improve OH tests

[mauricedesaxe]

Why?

To ensure the quality and working state of the OffsetHelper while also making tests more understandable and easier to work on when needed.

How?

Note: The diff may seem big (or at least it does to me), but most of it is very similar refactorings that just repeat multiple times.

I first did a few small refactorings like:

• renaming the test file to have the .test.ts extension
• removing unused imports
• fixed the ts-ignores that existed
• removed the problematic .not.be.reverted (why is it problematic? see the issue description here)
• extracted parseEther("1.0")s into a constant

Then I went on to bigger work:

Improving autoOffset, autoRedeem and autoRetire related tests

I grouped each of these, as relevant to their tested method, in their own commit because they were quite similar. I took each test (where it made sense) and I:

• extracted the token cost in a constant within the scope of the test (for readability & where applicable)
• create chain state checks that verify whether token balances and BCT/NCT supply have changed accordingly
• I used proper messages in expect so when tests fail you know where it failed
• I structured tests and commented on each section so it is as easy to understand as possible

Follow-up

I feel like the current improvements already take this a long way, but there are other things that I want to do. The problem is that this is already a lengthy PR and I don't want to make it even harder to review.

Some of the things I want to do in a follow-up PR:

• research & implement a good way to check the state of tco2 supply in OH tests
• add test failure messages in deposit(), withdraw() and particularly swap() tests
• extract the wrapping of matic in the should retire using a WMATIC swap and NCT redemption test to beforeAll scope