JetDiagnosticsTest migrated to TestGenerator

- test data files renamed from *.jet to *.kt

compiler/testData/diagnostics/tests/dataFlow/WhenSubject.kt

@@ -0,0 +1,10 @@
+trait Expr
+class BinOp(val operator : String) : Expr
+
+fun test(e : Expr) {
+    if (e is BinOp) {
+        when (e.operator) {
+            else -> 0
+        }
+    }
+}