chore(examples): add SetRealm context for testing on mgroups (#4201)

This fixes the failing tests for this package realm. 

By setting `testing.SetRealm(std.NewCodeRealm("gno.land/r/test/test"))`,
it allows the testing suite to configure the proper realm context needed
for the new cross/crossing constraints.

Author: n2p5

examples/gno.land/p/n2p5/mgroup/mgroup_test.gno

@@ -11,6 +11,7 @@ import (
 
 func TestManagedGroup(t *testing.T) {
 	t.Parallel()
+	testing.SetRealm(std.NewCodeRealm("gno.land/r/test/test"))
 
 	u1 := testutils.TestAddress("u1")
 	u2 := testutils.TestAddress("u2")