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

[n2p5]

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.

[Gno2D2]

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[n2p5]

context: #4060 broke a bunch of tests due to significant changes, and I wanted to fix the tests under my namespace. I'm starting with fixing my packages, then I'll move on to my realms.

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[leohhhn]

same as #4200 (comment)