test: run the unit test suite in parallel

[mehcode]

Before

❯ time cargo nextest run

...

Executed in   86.17 secs    fish           external
   usr time   30.24 secs    0.00 micros   30.24 secs
   sys time   11.48 secs  614.00 micros   11.48 secs

After

❯ time cargo nextest run

...

Executed in   20.07 secs    fish           external
   usr time   38.50 secs    0.00 micros   38.50 secs
   sys time   15.23 secs  569.00 micros   15.23 secs