JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/backingField/WriteNonexistentDeclaredInHigher.kt

@@ -0,0 +1,7 @@
+val y = 1
+
+class A() {
+    {
+        <!INACCESSIBLE_BACKING_FIELD!>$y<!> = 1
+    }
+}