CI: Migrate `cargo test` to `cargo-nextest`

[cmichi]

Motivation

cargo-nextest can be used instead of cargo test. It runs the tests as well, but is more performant. We should replace our usage of cargo test with it.

ToDo

• [x] Install cargo-nextest in the Docker image that use in our CI. The image is called ink-ci-linux, you can find the Dockerfile here. Just add a cargo install there.
• [x] Once merged you need to rebuild the Docker image. This is done every night. You can also trigger it by hand here, by click the play button: If you don't see a play button, ask in the "ink! <> CI/CD" channel. If the last build failed (as in this picture), ask the current 🦖 for help before triggering a rebuild. Then something keeps the build from succeeding.
• [ ] Once the Docker build succeeded you can create an ink! PR that replaces cargo test in our .gitlab-ci.yml with cargo-nextest invocations.
• [ ] Check if our GitHub actions under .github/ also need to be migrated.

Follow-up

In case this ticket worked out good and we don't see issues: please create a follow-up issue for cargo-contract.

[cmichi]

I'm closing this PR and the issue as the results are inconclusive, we might revisit after https://github.com/paritytech/ink/issues/1520.