test(tests/lean/run/partial_explicit): add test for '@@' operator

tests/lean/run/partial_explicit.lean

@@ -0,0 +1,18 @@
+section
+variables a b c : nat
+variable  H1 : a = b
+variable  H2 : a + b + a + b = 0
+
+example : b + b + a + b = 0 :=
+  @@eq.rec_on (λ x, x + b + a + b = 0) H1 H2
+end
+
+section
+variables (f : Π {T : Type₁} {a : T} {P : T → Prop}, P a → Π {b : T} {Q : T → Prop}, Q b → Prop)
+variables (T : Type₁) (a : T) (P : T → Prop) (pa : P a)
+variables (b : T) (Q : T → Prop) (qb : Q b)
+
+check @f T a P pa b Q qb -- Prop
+check @@f P pa Q qb      -- Prop
+check f pa qb            -- Prop
+end