test: make current time testable

[Komyyy]

This PR replaces DateTime.now with ZulipBinding.utcNow.
Also, this PR reduces some redundant arguments for current time; these arguments are not needed because we can use withClock for testing.

[chrisbobbe]

Thanks! fa5dfd5 added ZulipBinding.utcNow; can we use that, or is there a need to add a new field?

[PIG208]

I believe this is just a timezone conversion away from what we have in main. The part about replacing DateTime.now with the ZulipBinding equivalent is a nice follow-up we mentioned in 0c03009.

[chrisbobbe]

So it is something we want, separately from utcNow? If so, Komyyy let's put the two fields side-by-side and make their dartdocs more parallel/similar, with bidirectional "see also" notes that help the reader decide which one to use.

[Komyyy]

So it is something we want, separately from utcNow? If so, Komyyy let's put the two fields side-by-side and make their dartdocs more parallel/similar, with bidirectional "see also" notes that help the reader decide which one to use.

I wasn't aware that this function had been added.
The ZulipBinding.now function can be replaced with DateTime.toLocal and ZulipBinding.utcNow.
Therefore, I will update this PR to replace DateTime.now with ZulipBinding.utcNow.