test: refactor crumb test to standard path

[ivokub]

We have API tests in internal/backend/circuits package. Move this single test to expected path.

---

Note

Moves the isCrumb test from test/ into internal/backend/circuits using registry-based cases and removes the old test.

• Tests:
  • Add internal/backend/circuits/crumb.go:
    • Define isCrumbCircuit and register cases via addNewEntry:
      • "isCrumb/case=good" with valid crumbs []frontend.Variable{0,1,2,3}.
      • "isCrumb/case=bad" with randomized invalid values (math/rand/v2).
  • Remove legacy test/api_assertions_test.go that used testing.T-based assertions for AssertIsCrumb.

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