Add reproduction case for #1980

[Lucccyo]

This PR aims to reproduce #1980.

[xvw]

Should the test be fixed ?
If so, can you add a FIXME: comment to explain future fixes?

[Lucccyo]

The remaining issue with the Windows CI is not very clear to me.
What I understood is that the path styles are very different on Linux and Windows, and it fails with that :

$ ocamlmerlin single locate -position 11:16 -look-for implementation -filename test.ml < test.ml | jq .value
   {
-    "file": "$TESTCASE_ROOT/test.ml",
+    "file": "D:\\a\\merlin\\merlin\\_build\\.sandbox\\9ac33dbba3714f9ff12e42444b1c602e\\default\\tests\\test-dirs\\test.ml",
     "pos": {
       "line": 4,
       "col": 6

What do you think about this @voodoos @xvw ?
To fix that, I will add a jq .pos, but I am interested in the answer because it should not result in a path error, and I might have made an error.

[voodoos]

Path sanitization is broken on windows. We should fix it someday. Meanwhile you can just disable that test on windows by adding it to the appropriate list in the tests dune file: https://github.com/ocaml/merlin/blob/4ab34aba1ac52534e313d7b9e6e11db77ae2bf09/tests/test-dirs/dune