test: add a test for project-wide occurrences

Requires ocaml-index

tests/test-dirs/occurrences/project-wide/pwo-basic.t

@@ -0,0 +1,65 @@
+  $ cat >lib.ml <<'EOF'
+  > let foo = "bar"
+  > let () = print_string foo
+  > EOF
+
+  $ cat >main.ml <<'EOF'
+  > let () = print_string Lib.foo
+  > EOF
+
+  $ ocamlc -bin-annot -bin-annot-occurrences -c lib.ml main.ml
+
+  $ ocaml-index aggregate main.cmt lib.cmt
+  $ ocaml-index dump project.ocaml-index
+  2 uids:
+  {uid: Stdlib.312; locs:
+     "print_string": File "lib.ml", line 2, characters 9-21;
+     "print_string": File "main.ml", line 1, characters 9-21
+   uid: Lib.0; locs:
+     "foo": File "lib.ml", line 1, characters 4-7;
+     "foo": File "lib.ml", line 2, characters 22-25;
+     "Lib.foo": File "main.ml", line 1, characters 22-29
+   }, 0 approx shapes: {}, and shapes for CUS .
+
+  $ $MERLIN single occurrences -scope project -identifier-at 1:28 \
+  > -index-file project.ocaml-index \
+  > -filename main.ml <main.ml
+  {
+    "class": "return",
+    "value": [
+      {
+        "file": "$TESTCASE_ROOT/lib.ml",
+        "start": {
+          "line": 1,
+          "col": 4
+        },
+        "end": {
+          "line": 1,
+          "col": 7
+        }
+      },
+      {
+        "file": "$TESTCASE_ROOT/lib.ml",
+        "start": {
+          "line": 2,
+          "col": 22
+        },
+        "end": {
+          "line": 2,
+          "col": 25
+        }
+      },
+      {
+        "file": "$TESTCASE_ROOT/main.ml",
+        "start": {
+          "line": 1,
+          "col": 26
+        },
+        "end": {
+          "line": 1,
+          "col": 29
+        }
+      }
+    ],
+    "notifications": []
+  }