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

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

@@ -0,0 +1,6 @@
+class Flower() {
+
+    var minusOne: Int = 1
+        get() = field + 1
+        set(n: Int) { field = n - 1 }
+}