Fix flaky timestamp tests

[frangio]

They are due to an how OpenZeppelin Test Helpers implements time.increaseTo. We should prefer Hardhat's own network helpers (eventually we should migrate the rest of the test suite).

I also included a small refactor of .then in the Governor helper.

Fixes #4049

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[Amxx]

Awesome !

[frangio]

The test suite is going a lot slower than before. Is it due to this PR?

[frangio]

Test suite took twice the time it usually takes. I'm disabling gas reporting to see if it helps.

[Amxx]

Test suite took twice the time it usually takes. I'm disabling gas reporting to see if it helps.

gas report definitelly making all tests WAY slower

[frangio]

Gas Reporter	OZ Helpers	Nomic Helpers
on	20m	35m
off	10m	22m

[Amxx]

It made sens that d79c2ec would fix the issue of flaky timestamps, but I'm not sure how we expect the current version of the PR to do anything since the behavior for timestamps in helper/time.js is no longer part of the diff.

[frangio]

@Amxx I'm just trying things out to debug the test time degradation.