adding test case with lagged exogenous variable (currently fails)

tests/auxiliary_variables/test2.mod

@@ -0,0 +1,14 @@
+var y;
+varexo e;
+
+model;
+y = e + e(-1);
+end;
+
+initval;
+y = 1;
+end;
+
+steady;
+
+model_diagnostics;