Remove reliance of tests on Cats Effect, to enable Scala Native release for sttp client module

[keynmol]

Most tests operate within or via Future anyways, so we remove the level of indirection and express as much as possible in terms of Future, removing the need for CE dependency in a lot of places, but especially on Native. I got sttpClient4Native4/test to pass locally, which is all I care about really.

Note that CE release on Native 0.5 is imminent, but if it's delayed, this might be an option to get native builds of sttp modules out (and may be others?).

Getting a native tapir-sttp-client module is my only goal here, so I don't care if it's achieved by any other means – you can consider this PR if you think it has merit on its own.

not a Draft as I want to get a CI run out of it.

[adamw]

I'm fine with refactoring the tests to use Futures instead of c-e, though then we'd have to refactor all of them so that there's a single version of ClientTests. Do you know when c-e with support of Native will get released?

Although, even if we'll get a 0.5 release, who knows what will be the timeline for 0.6 and onwards ;)

[keynmol]

though then we'd have to refactor all of them so that

I've attempted to do just that in f7e5095

who knows what will be the timeline for 0.6 and onwards

That's a possibility for sure, but IIRC Wojciech was planning a 1.x release after 0.5...

Do you know when c-e with support of Native will get released?

It's on hold until SN 0.5.8 I think.