HLS source distribution is missing extra-source-files necessary for testing

[fendor]

Vs:

> ls plugins/hls-semantic-tokens-plugin/test/testdata/ | wc -l
52

In particular, we are missing all *.expected files. We should test that we can run the tests of the source distribution produced by cabal sdist.

[fendor]

Closed by #4105

[fendor]

We fixed this situation in #4105 and #4107, but we should add a CI check that this can't happen again. Potentially, the workflow for running tests should be as follows: cabal sdist > tar > cabal test. Iirc, haskell-ci generates this workflow.

[michaelpj]

Relevant: haskell/cabal#1141