JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/controlFlowAnalysis/kt843.kt

@@ -0,0 +1,8 @@
+//KT-843 Don't highlight incomplete variables as unused
+
+package kt843
+
+fun main(args : Array<String>) {
+    // Integer type
+    val<!SYNTAX!><!> // this word is grey, which looks strange
+}