ci: use nextest instead of cargo test

[link2xt]

See https://nexte.st/ for the documentation.

[link2xt]

Maybe not worth it given that it takes some time to install and building the test binaries takes longer than actually testing.

[link2xt]

Actually, it only takes 3 seconds to install, so does not hurt given that at least locally it saves more than that.

It reports individual test times, which look strange on CI:

        PASS [  13.892s] deltachat accounts::tests::test_accounts_share_translations
        PASS [  24.466s] deltachat accounts::tests::test_accounts_sorted
        PASS [  49.580s] deltachat accounts::tests::test_accounts_ids_unique_increasing_and_persisted

The whole Rust test suite which normally takes 12 seconds on github actions runs for 7 minutes and some tests which should take less than a second pass in tens of seconds. GitHub Actions probably pauses them somehow.