FIR: suppress VARIABLE_EXPECTED for error reference

compiler/testData/diagnostics/tests/incompleteCode/incompleteAssignment.fir.kt

@@ -7,7 +7,7 @@ fun sum(a : IntArray) : Int {
     // Write your solution here
     <!UNRESOLVED_REFERENCE!>res<!> = 0
     for (e in a)
-        <!UNRESOLVED_REFERENCE, VARIABLE_EXPECTED!>res<!> +=<!SYNTAX!><!>
+        <!UNRESOLVED_REFERENCE!>res<!> +=<!SYNTAX!><!>
 <!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY{LT}!>}<!>
 fun main() {
     test(0)