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

compiler/testData/diagnostics/tests/when/NonExhaustiveWarningFalse.fir.kt

@@ -0,0 +1,13 @@
+// Base for KT-6227
+enum class X { A, B, C, D }
+
+fun foo(arg: X): String {
+    val res: String
+    when (arg) {
+        X.A -> res = "A"
+        X.B -> res = "B"
+        X.C -> res = "C"
+        X.D -> res = "D"
+    }
+    return res
+}