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

compiler/testData/diagnostics/tests/backingField/InitCustomSetter.fir.kt

@@ -0,0 +1,30 @@
+class My(val v: Int) {
+    // Ok: setter is just private
+    var x: Int
+        private set
+
+    var y: Int
+        set(arg) { field = arg }
+
+    var z: Int
+        set(arg) { field = arg }
+
+    // Ok: initializer available
+    var w: Int = v
+        set(arg) { field = arg }
+
+    // Ok: no backing field
+    var u: Int
+        get() = w
+        set(arg) { w = 2 * arg }
+
+    constructor(): this(0) {
+        z = v        
+    }
+
+    init {
+        x = 1
+        y = 2
+        u = 3
+    }
+}