JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/OverridingVarByVal.kt

@@ -0,0 +1,15 @@
+open class Var() {
+  open var v : Int = 1
+}
+
+trait VarT {
+  var v : Int
+}
+
+class Val() : Var(), VarT {
+  override <!VAR_OVERRIDDEN_BY_VAL!>val<!> v : Int = 1
+}
+
+class Var2() : Var() {
+  override var v : Int = 1
+}