test(frontends/lean): add 'l = nil' test

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

tests/lean/eq2.lean

@@ -0,0 +1,7 @@
+Variable List : Type -> Type
+Variable nil  {A : Type} : List A
+Variable cons {A : Type} (head : A) (tail : List A) : List A
+Variable l : List Int.
+Check l = nil.
+Set pp::implicit true
+Check l = nil.

tests/lean/eq2.lean.expected.out

@@ -0,0 +1,9 @@
+  Set: pp::colors
+  Set: pp::unicode
+  Assumed: List
+  Assumed: nil
+  Assumed: cons
+  Assumed: l
+l = nil : Bool
+  Set: lean::pp::implicit
+eq::explicit (List ℤ) l (nil::explicit ℤ) : Bool