test: drop duplicate tenure-extend test

[simone-stacks]

check_tenure_extend_no_burn_change_during_read_count and check_tenure_extend_unsupported_cause have byte-identical bodies (sha256 3c41f0aa…1f2a42e). The first test's name promises a discrimination the body never delivers — both hit the same cause = ExtendedRuntime rejection path. Deleting it provably preserves the cargo-mutants kill count.

Result of a first smoke-test of the quality-agent.

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