CI: try nextest

[TriplEight]

An attempt to switch to cargo-nextest.

Note it doesn't support doctests so they should be run separately.
@dvdplm @niklasad1 could you suggest a good way of getting the list of doctests please?

---

Oh lovely.
So nextest with some cache (on the second run) performed like this

instead of a regular cargo test

it's worth mentioning though that without cache it takes another ~3m30s to install the tool. Thankfully, there's a cached cargo-install.

[niklasad1]

cool, it's 1 minute faster than cargo test

[niklasad1]

@TriplEight

hrm, I would like at least one approval from the tools team before merging any PRs to this repo in future.

We rely heavily on doc tests and we need have to doc tests enabled to be sure that we don't miss any tests.

I'll create a follow--up PR to add that back

[TriplEight]

@niklasad1 sorry for that, noted.
https://github.com/paritytech/jsonrpsee/blob/master/.github/workflows/ci.yml#L46 or
https://github.com/paritytech/jsonrpsee/blob/master/.gitlab-ci.yml#L99 I missed that these are not running all the doctests.
Anyway, doctests usually do not require compilation and can be safely separated to another job.

The follow-up issue to work on this repo is here https://github.com/paritytech/ci_cd/issues/333
And this is how we'll prepare to the nextest deployment next time https://github.com/paritytech/ci_cd/issues/334
Please feel free to add your ideas.