add failing test demonstrating issue with parser

Author: zth

analysis/tests/src/CompletionFunctionArguments.res

@@ -53,3 +53,7 @@ let someFnTakingVariant = (
 
 // let _ = someFnTakingVariant(~configOpt2=O)
 //                                          ^com
+
+// This below demonstrates an issue when what you're completing is the _last_ labelled argument, and there's a unit application after it. The parser wrongly merges the unit argument as the expression of the labelled argument assignment, where is should really let the trailing unit argument be, and set a %rescript.exprhole as the expression of the assignment, just like it normally does.
+// let _ = someFn(~isOff=, ())
+//                       ^com

analysis/tests/src/expected/CompletionFunctionArguments.res.txt

@@ -140,3 +140,16 @@ Completable: Value[someFnTakingVariant](~configOpt2=O)
     "documentation": null
   }]
 
+Complete src/CompletionFunctionArguments.res 57:25
+posCursor:[57:25] posNoWhite:[57:24] Found expr:[57:11->57:30]
+Pexp_apply ...[57:11->57:17] (~isOff57:19->57:24=...[57:27->57:29])
+Completable: CnamedArg(Value[someFn], "", [isOff])
+Found type for function (~isOn: bool, ~isOff: bool=?, unit) => string
+[{
+    "label": "isOn",
+    "kind": 4,
+    "tags": [],
+    "detail": "bool",
+    "documentation": null
+  }]
+