JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/regressions/kt455.kt

@@ -0,0 +1,10 @@
+//KT-455 Do not repeat errors in definite assignment checks
+
+package kt455
+
+fun foo() {
+    val a: Int
+    doSmth(<!UNINITIALIZED_VARIABLE!>a<!>)   //error
+    doSmth(a)   //no repeat error
+}
+fun doSmth(<!UNUSED_PARAMETER!>i<!>: Int) {}