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.