JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/controlFlowAnalysis/kt610.kt

@@ -0,0 +1,10 @@
+//KT-610 Distinguish errors 'unused variable' and 'variable is assigned but never accessed'
+
+package kt610
+
+fun foo() {
+    var <!UNUSED_VARIABLE!>j<!> = 9  //'unused variable' error
+
+    var <!ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE!>i<!> = 1  //should be an error 'variable i is assigned but never accessed'
+    i = <!UNUSED_VALUE!>2<!>
+}