Add tests for universe bumping of `gset`.

tests/universes.ref

+gmap Z Z : Set
+     : Set
+gset Z : Set
+     : Set

tests/universes.v

+From stdpp Require Import gmap.
+
+(** Make sure that [gmap] and [gset] do not bump the universe. Since [Z : Set],
+the types [gmap Z Z] and [gset Z] should have universe [Set] too. *)
+
+Check (gmap Z Z : Set).
+Check (gset Z : Set).