test(milestones): assert MilestoneClosed on allocate and cancel after cancellation (#160, #161)

[ghzhost]

Summary

Adds regression unit tests verifying that both cancel_milestone() and allocate() return Error::MilestoneClosed when invoked on an already-cancelled milestone.

• Double cancel (milestones: cancel_milestone rejecting a double-cancel (MilestoneClosed) has no test #160): verifies calling cancel_milestone a second time on the same milestone_id returns Err(Ok(Error::MilestoneClosed)).
• Allocate on closed (milestones: allocate() on an already-closed milestone (MilestoneClosed) has no test #161): verifies calling allocate on a cancelled milestone returns Err(Ok(Error::MilestoneClosed)).

Closes #160
Closes #161

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