Add a test illustrating issue 1842

tests/test-dirs/locate/issue1842.t

@@ -0,0 +1,33 @@
+  $ cat >dune-project <<'EOF'
+  > (lang dune 3.0)
+  > EOF
+
+  $ mkdir real_source
+
+  $ cat >real_source/pparse.ml <<'EOF'
+  > let x = "Hello"
+  > EOF
+
+  $ mkdir source
+
+  $ cat >source/main.ml <<'EOF'
+  > let () = print_endline Pparse.x 
+  > EOF
+
+  $ cat >source/dune <<'EOF'
+  > (rule (copy# ../real_source/pparse.ml pparse.ml))
+  > (executable (name main))
+  > EOF
+
+
+  $ dune build @check
+  $ dune exec ./source/main.exe
+  Hello
+
+  $ $MERLIN single locate -look-for ml -position 1:30 \
+  > -filename source/main.ml <source/main.ml
+  {
+    "class": "return",
+    "value": "'Dune__exe.Pparse.x' seems to originate from 'Pparse' whose ML file could not be found",
+    "notifications": []
+  }