JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/regressions/kt174.kt

@@ -0,0 +1,7 @@
+// KT-174 Nullability info for extension function receivers
+trait Tree {}
+
+fun Any?.TreeValue() : Tree {
+  if (this is Tree) return this
+    throw Exception()
+}