JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/declarationChecks/kt2142.kt

@@ -0,0 +1,8 @@
+//KT-2142 function local classes do not work
+
+package a
+
+fun foo() {
+    class Foo() {}
+    Foo() // Unresolved reference Foo
+}