fix(tests/lean): make sure pretty print and parse test passes

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

tests/lean/using_bug1.lean

@@ -1,5 +1,8 @@
 variables a b c d : Nat
 axiom H : a + (b + c) = a + (b + d)
+
+set_option pp::implicit true
+
 using Nat
 check add_succr a
 

tests/lean/using_bug1.lean.expected.out

@@ -5,6 +5,7 @@
   Assumed: c
   Assumed: d
   Assumed: H
+  Set: lean::pp::implicit
   Using: Nat
-Nat::add_succr a : ∀ b : ℕ, a + (b + 1) = a + b + 1
+Nat::add_succr a : ∀ b : ℕ, @eq ℕ (a + (b + 1)) (a + b + 1)
   Proved: mul_zerol2