test: add new_frontend command test

tests/lean/run/newfrontend1.lean

@@ -0,0 +1,17 @@
+def x := 1
+
+new_frontend
+
+#check x
+
+variables {α : Type}
+
+def f (a : α) : α :=
+a
+
+def tst (xs : List Nat) : Nat :=
+xs.foldl (init := 10) (· + ·)
+
+#check tst [1, 2, 3]
+
+#check tst