Get rid of the hack needed by the "Expression is `_`" test

Currently, the case-split plugin **_test_** that ensures that the plugin works on `case _ :: Foo of` (where the `_` causes an error, which is fundamental to the test) needs a hack, which consists in using this function:

https://github.com/haskell/haskell-language-server/blob/5ce2a847d255ce6f420da49771711a89eee78541/plugins/hls-case-split-plugin/test/Main.hs#L171-L179

instead of the exiting util:

https://github.com/haskell/haskell-language-server/blob/5ce2a847d255ce6f420da49771711a89eee78541/hls-test-utils/src/Test/Hls/Util.hs#L281-L287

For details, see https://github.com/haskell/haskell-language-server/pull/5014#discussion_r3757061984

This is a hack _**for the test**_, not for the product code, hence no bug label.

Originally considered during [my GSoC 2026 project](https://gist.github.com/Aster89/69a57c364ec2d0cc89f474cc3d194371#future-work--stretch-goals).

Start with the linked PR discussion, then compare the local helper in plugins/hls-case-split-plugin/test/Main.hs with the existing utility in hls-test-utils/src/Test/Hls/Util.hs. Run the case-split plugin test and confirm the test for `case _ :: Foo of` still passes without the test-specific hack.

Written by the indexing model from the issue text.