[test] change to sequential

It might be that notifications get mixed up when running parallel
test (as they use the same port)
6 jobs for 238-dev-async-job-worker in 32 minutes and 56 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Cabal
passed #16171
cabal

00:12:44

failed #16170
cabal

00:00:18

failed #16166
cabal

00:00:17

 
  Stack
passed #16167
stack

00:05:18

 
  Bench
manual #16168
allowed to fail manual
bench
 
  Test
passed #16169
test

00:14:53