CI jobs of plugins' test suites got stuck randomly

[berberman]

#1606 and #1612 moved plugins' test suites and created standalone CI jobs for them. However, according to the runs for recent PRs, it seemd that they got stuck randomly:

https://github.com/haskell/haskell-language-server/runs/2197372198
https://github.com/haskell/haskell-language-server/runs/2206602816
https://github.com/haskell/haskell-language-server/runs/2206604842

[pepeiborra]

You need to make sure to build with --threaded and have 2 or more capabilities, or we could just fix #727