Add Change Type Signature test suite to CI

[pepeiborra]

Ooops, we forgot to add the test suite to CI

https://github.com/haskell/haskell-language-server/blob/0d4f98eff2acc4e5df2d2bdd6111b04f91d1ec38/.github/workflows/test.yml

Originally posted by @pepeiborra in #2660 (comment)