Flaky test suite: progress reporting (func-test)

[pepeiborra]

This test suite fails often with an unexpected progress token error:

FAIL (1.11s)
      test/functional/Progress.hs:105:
      Unexpected ProgressTextToken "47"

https://github.com/haskell/haskell-language-server/runs/4754928207?check_suite_focus=true
https://github.com/haskell/haskell-language-server/runs/4754788726?check_suite_focus=true
https://github.com/haskell/haskell-language-server/runs/4754639502?check_suite_focus=true
https://github.com/haskell/haskell-language-server/runs/4754227465?check_suite_focus=true