[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/operatorsOverloading/plusAssignOnProperty.fir.kt

@@ -0,0 +1,19 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class C {
+    val c: C = C()
+}
+
+operator fun C.plus(a: Any): C = this
+operator fun C.plusAssign(a: Any) {}
+
+class C1 {
+    var c: C = C()
+}
+
+fun test() {
+    val c = C()
+    c.c += ""
+    var c1 = C1()
+    <!ASSIGN_OPERATOR_AMBIGUITY!>c1.c += ""<!>
+}