test: fix brittle structure instance completion test (#6127)

#5835 contains a brittle test that uses an FVar ID, which caused a
failure on master. This PR changes that test to use a declaration
instead.

tests/lean/interactive/completionStructureInstance.lean

@@ -92,5 +92,7 @@ example (s : S) : S := { s with   : S } -- All field completions expected
 example (s : S) : S := { s with f  } -- All field completions matching `f` expected
                                --^ textDocument/completion
 
-example (aLongUniqueIdentifier : Nat) : Std.HashSet Nat := { aLongUniqueIdentifier } -- Identifier completion matching `aLongUniqueIdentifier`
-                                                                                --^ textDocument/completion
+def aLongUniqueIdentifier := 0
+
+example : Std.HashSet Nat := { aLongUniqueIdentifier } -- Identifier completion matching `aLongUniqueIdentifier`
+                                                  --^ textDocument/completion

tests/lean/interactive/completionStructureInstance.lean.expected.out

@@ -598,15 +598,15 @@
     "cPos": 0}}],
  "isIncomplete": true}
 {"textDocument": {"uri": "file:///completionStructureInstance.lean"},
- "position": {"line": 94, "character": 82}}
+ "position": {"line": 96, "character": 52}}
 {"items":
  [{"sortText": "0",
    "label": "aLongUniqueIdentifier",
-   "kind": 6,
+   "kind": 21,
    "data":
    {"params":
     {"textDocument": {"uri": "file:///completionStructureInstance.lean"},
-     "position": {"line": 94, "character": 82}},
-    "id": {"fvar": {"id": "_uniq.1028"}},
+     "position": {"line": 96, "character": 52}},
+    "id": {"const": {"declName": "aLongUniqueIdentifier"}},
     "cPos": 0}}],
  "isIncomplete": true}