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

compiler/testData/diagnostics/tests/sealed/TreeWhenFunctional.fir.kt

@@ -0,0 +1,11 @@
+sealed class Tree {
+    object Empty: Tree()
+    class Leaf(val x: Int): Tree()
+    class Node(val left: Tree, val right: Tree): Tree()
+
+    fun max(): Int = when(this) {
+        is Empty -> -1
+        is Leaf  -> this.x
+        is Node  -> this.left.max()
+    }
+}