feat: Use `cargo nextest` for more test parallelism

The cargo test command cannot parallelize across crates and it makes tests slower than necessary:

$ time cargo test --all
...
real	0m8,449s
user	1m24,149s
sys	0m37,558s

Comparing with nextest the difference is clear:

$ time cargo nextest run --all
...
real	0m2,898s
user	0m5,922s
sys	0m2,079s