Added fake override search in unused symbol inspection #KT-13288 Fixed

idea/testData/inspections/unusedSymbol/property/triangle.kt

@@ -0,0 +1,17 @@
+// See KT-13288
+
+interface Inter {
+    fun something(): String
+}
+
+abstract class Abstract {
+    fun something() = "hi"
+}
+
+class Test: Abstract(), Inter {
+}
+
+fun main(args: Array<String>) {
+    val x: Inter = Test()
+    x.something()
+}