JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/regressions/kt600.kt

@@ -0,0 +1,8 @@
+//KT-600 Problem with 'sure' extension function type inference
+
+fun <T : Any> T?._sure() : T { if (this != null) return this else throw NullPointerException() }
+
+fun test() {
+    val i : Int? = 10
+    val <!UNUSED_VARIABLE!>i2<!> : Int = i._sure() // inferred type is Int? but Int was excepted
+}