JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/scopes/kt421Scopes.kt

@@ -0,0 +1,8 @@
+// http://youtrack.jetbrains.net/issue/KT-421
+// KT-421 Strange 'unresolved' bug with backing fields
+
+class A() {
+    val c = 1
+    val a = <!UNINITIALIZED_VARIABLE!>b<!>
+    val b = $c  // '$c' is unresolved
+}